Wafer-level heterogeneous integration of MEMS actuators Stefan Braun

Wafer-level heterogeneous integration of MEMS actuators Stefan Braun
Wafer-level heterogeneous
integration of MEMS actuators
Stefan Braun
MICROSYSTEM TECHNOLOGY LABORATORY
SCHOOL OF ELECTRICAL ENGINEERING
ROYAL INSTITUTE OF TECHNOLOGY
ISBN 978-91-7415-493-1
ISSN 1653-5146
TRITA-EE 2010:002
Submitted to the School of Electrical Engineering
KTH—Royal Institute of Technology, Stockholm, Sweden,
in partial fulfillment of the requirements for the degree of Doctor of Philosophy
Stockholm 2010
ii
Wafer-level heterogeneous integration of MEMS actuators
Copyright © 2010 by Stefan Braun
All rights reserved to the summary part of this thesis, including all pictures and
figures. No part of this publication may be reproduced or transmitted in any form or
by any means, without prior permission in writing from the copyright holder.
The copyrights for the appended journal papers belong to the publishing houses of
the journals concerned. The copyrights for the appended manuscripts belong to their
authors.
Printed by Universitetsservice US AB, Stockholm 2010.
Thesis for the degree of Doctor of Philosophy at the Royal Institute of Technology,
Stockholm, Sweden, 2010.
ABSTRACT
iii
Abstract
This thesis presents methods for the wafer-level integration of shape memory alloy
(SMA) and electrostatic actuators to functionalize MEMS devices. The integration
methods are based on heterogeneous integration, which is the integration of different
materials and technologies. Background information about the actuators and the
integration method is provided.
SMA microactuators offer the highest work density of all MEMS actuators, however, they are not yet a standard MEMS material, partially due to the lack of proper
wafer-level integration methods. This thesis presents methods for the wafer-level heterogeneous integration of bulk SMA sheets and wires with silicon microstructures.
First concepts and experiments are presented for integrating SMA actuators with
knife gate microvalves, which are introduced in this thesis. These microvalves feature
a gate moving out-of-plane to regulate a gas flow and first measurements indicate outstanding pneumatic performance in relation to the consumed silicon footprint area.
This part of the work also includes a novel technique for the footprint and thickness
independent selective release of Au-Si eutectically bonded microstructures based on
localized electrochemical etching.
Electrostatic actuators are presented to functionalize MEMS crossbar switches,
which are intended for the automated reconfiguration of copper-wire telecommunication networks and must allow to interconnect a number of input lines to a number
of output lines in any combination desired. Following the concepts of heterogeneous
integration, the device is divided into two parts which are fabricated separately and
then assembled. One part contains an array of double-pole single-throw S-shaped actuator MEMS switches. The other part contains a signal line routing network which
is interconnected by the switches after assembly of the two parts. The assembly is
based on patterned adhesive wafer bonding and results in wafer-level encapsulation
of the switch array. During operation, the switches in these arrays must be individually addressable. Instead of controlling each element with individual control lines,
this thesis investigates a row/column addressing scheme to individually pull in or pull
out single electrostatic actuators in the array with maximum operational reliability,
determined by the statistical parameters of the pull-in and pull-out characteristics of
the actuators.
Keywords:
Microelectromechanical systems, MEMS, silicon, wafer-level, integration, heterogeneous integration, transfer integration, packaging, assembly, wafer
bonding, adhesive bonding, eutectic bonding, release etching, electrochemical etching, microvalves, microactuator, Shape Memory Alloy, SMA, NITINOL, TiNi, NiTi,
cold-state reset, bias spring, stress layers, crossbar switch, routing, switch, switch
array, electrostatic actuator, S-shaped actuator, zipper actuator, addressing, transfer
stamping, blue tape
Stefan Braun, [email protected]
Microsystem Technology Laboratory, School of Electrical Engineering
KTH—Royal Institute of Technology, SE-100 44 Stockholm, Sweden
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Wafer-level heterogeneous integration of MEMS actuators
... I have not filled this volume with pompous rhetoric, with bombast and
magnificent words, or with the unnecessary artice with which so many
writers gild their work. I wanted nothing extranous to ornament my
writing, for it has been my purpose that only the range of material and
the gravity of the subject should make it pleasing. ...
From “Il Principe”
Niccolò Machiavelli, 1469-1532
Translated by Peter Constantine, “The Prince - A new translation”
ABSTRACT
v
To my family
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Wafer-level heterogeneous integration of MEMS actuators
CONTENTS
vii
Contents
Abstract
iii
List of papers
ix
1 Introduction and structure
1
2 MEMS actuators
2.1 Common microactuation mechanisms . . . . . . . . . . . . . . . .
2.2 Shape Memory Alloy actuation . . . . . . . . . . . . . . . . . . .
2.2.1 Shape Memory Effects . . . . . . . . . . . . . . . . . . .
2.2.2 Actuation aspects of Titanium-Nickel alloys . . . . . . . .
2.3 Electrostatic actuation . . . . . . . . . . . . . . . . . . . . . . . .
2.3.1 Parallel-plate, comb-drive and curved-electrode actuators
2.3.2 S-shaped film actuators . . . . . . . . . . . . . . . . . . .
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3 Heterogeneous integration
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.1 Monolithic and hybrid integration . . . . . . . . . . . . . . .
3.1.2 Heterogeneous integration . . . . . . . . . . . . . . . . . . . .
3.2 Heterogeneous integration concepts and technologies . . . . . . . . .
3.2.1 Transfer/direct and wafer-to-wafer/chip-to-wafer integration .
3.2.2 Electrical interconnection . . . . . . . . . . . . . . . . . . . .
3.2.3 Wafer-bonding techniques . . . . . . . . . . . . . . . . . . . .
3.2.4 Releasing structures for actuation . . . . . . . . . . . . . . .
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4 Knife gate microvalves with bulk SMA microactuators
4.1 Integration of Titanium-Nickel shape memory alloy . . . .
4.2 Gas microvalves . . . . . . . . . . . . . . . . . . . . . . . .
4.2.1 Background . . . . . . . . . . . . . . . . . . . . . .
4.2.2 Knife gate microvalves . . . . . . . . . . . . . . . .
4.3 TiNi sheet actuated knife gate valves . . . . . . . . . . . .
4.3.1 Design . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.2 Fabrication - Integration of TiNi sheets . . . . . .
4.3.3 Results . . . . . . . . . . . . . . . . . . . . . . . .
4.3.4 Discussion . . . . . . . . . . . . . . . . . . . . . . .
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viii
Wafer-level heterogeneous integration of MEMS actuators
4.4
4.5
TiNi wire actuated knife gate valves . . . . .
4.4.1 Concept . . . . . . . . . . . . . . . . .
4.4.2 Fabrication - Integration of TiNi wires
4.4.3 Results . . . . . . . . . . . . . . . . .
4.4.4 Discussion . . . . . . . . . . . . . . . .
Outlook . . . . . . . . . . . . . . . . . . . . .
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5 Automated main distributing frames with S-shaped actuator switches 43
5.1 Switch units in automated main distributing frames . . . . . . . . . . 43
5.2 MEMS switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
5.2.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
5.2.2 The S-shaped actuator switch . . . . . . . . . . . . . . . . . . . 47
5.3 The MEMS crossbar switch . . . . . . . . . . . . . . . . . . . . . . . . 48
5.3.1 The routing network . . . . . . . . . . . . . . . . . . . . . . . . 48
5.3.2 The crosspoint switches . . . . . . . . . . . . . . . . . . . . . . 50
5.3.3 The integration . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
5.3.4 Individual switch addressing . . . . . . . . . . . . . . . . . . . . 50
5.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
5.5 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
6 Summaries of the Appended Papers
57
7 Conclusions
61
Acknowledgements
63
References
65
Paper reprints
79
LIST OF PAPERS
ix
List of papers
The presented thesis is based on the following journal papers:
1. Out of plane knife gate microvalves for controlling large gas flows
S. Haasl, S. Braun, A. S. Ridgeway, S. Sadoon, W. van der Wijngaart and
G. Stemme
IEEE/ASME Journal of Microelectromechanical Systems, vol. 15, no. 5, pp. 1281–
1288, Oct. 2006.
2. Wafer-scale manufacturing of bulk shape memory alloy microactuators based on
adhesive bonding of Titanium-Nickel sheets to structured silicon wafers
S. Braun, N. Sandström, G. Stemme and W. van der Wijngaart
IEEE/ASME Journal of Microelectromechanical Systems, accepted for publication
3. Design and wafer-level fabrication of SMA wire microactuators on silicon
D. Clausi, H. Gradin, S. Braun, J. Peirs, G. Stemme, D. Reynaerts and
W. van der Wijngaart
IEEE/ASME Journal of Microelectromechanical Systems, accepted for publication
4. Localized removal of the Au-Si eutectic bonding layer for the selective release of
microstructures
H. Gradin, S. Braun, G. Stemme and W. van der Wijngaart
IOP Journal of Micromechanics and Microengineering, vol. 19, no. 10, pp. 105014–
105023, Oct. 2009.
5. Single-chip MEMS 5×5 and 20×20 double-pole single-throw switch arrays for
automating telecommunication networks
S. Braun, J. Oberhammer and G. Stemme
IOP Journal of Micromechanics and Microengineering, vol. 18, no. 1, pp. 015014–
015025, Jan. 2008.
6. Row/Column addressing scheme for large electrostatic actuator MEMS switch
arrays and optimization of the operational reliability by statistical analysis
S. Braun, J. Oberhammer and G. Stemme
IEEE/ASME Journal of Microelectromechanical Systems, vol. 17, no. 5, pp. 1104–
1113, Oct. 2008.
The contribution of Stefan Braun to the different publications:
1
2
3
4
5
6
part of fabrication, all experiments, part of writing
major part of design, major parts of fabrication and experiments, all writing
part of design, fabrication, experiments and writing
part of design, fabrication and experiments, major part of writing
major part of design, all fabrication and experiments, major part of writing
major part of design, all fabrication and experiments, major part of writing
x
Wafer-level heterogeneous integration of MEMS actuators
The work has also been presented at the following international conferences:
1. Small footprint knife gate microvalves for large flow control
S. Braun, S. Haasl, S. Sadoon, A. S. Ridgeway, W. van der Wijngaart and
G. Stemme
The 13th IEEE Int. Conf. on Solid-state Sensors, Actuators and Microsystems
(TRANSDUCERS), Seoul, Korea, June 2005, pp. 329–332.
2. MEMS single chip microswitch array for re-configuration of telecommunication
networks
S. Braun, J. Oberhammer and G. Stemme
Proceedings of the 36th European Microwave Conference (EUMW), Manchester,
UK, Sep. 2006, pp. 811–814.
3. MEMS single-chip 5x5 and 20x20 double-switch arrays for telecommunication
networks
S. Braun, J. Oberhammer and G. Stemme
20th IEEE Int. Conf. on Micro Electro Mechanical Systems (MEMS), Kobe,
Japan, Jan. 2007, pp. 811–814.
4. Smart individual switch addressing of 5x5 and 20x20 MEMS double-switch arrays
S. Braun, J. Oberhammer and G. Stemme
The 14th IEEE Int. Conf. on Solid-state Sensors, Actuators and Microsystems
(TRANSDUCERS), Lyon France, June 2007, pp. 153–156.
5. Robust trimorph bulk SMA microactuators for batch manufacturing and integration
S. Braun, T. Grund, S. Ingvarsdottír, W. van der Wijngaart, M. Kohl and
G. Stemme
The 14th IEEE Int. Conf. on Solid-state Sensors, Actuators and Microsystems
(TRANSDUCERS), Lyon, France, June 2007, pp.2191–2194.
6. Wafer-scale manufacturing of robust trimorph bulk SMA microactuators
N. Sandström, S. Braun, T. Grund, G. Stemme, M. Kohl and W. van der Wijngaart
Proceedings of the 11th Int. Conf. on new Actuators (ACTUATOR), Bremen,
Germany, June 2008, pp. 382–385.
7. Microactuation utilizing wafer-level integrated SMA wires
D. Clausi, H. Gradin, S. Braun, J. Peirs, G. Stemme, D. Reynaerts and
W. van der Wijngaart
22nd IEEE Int. Conf. on Micro Electro Mechanical Systems (MEMS), Sorrento,
Italy, Jan. 2009, pp. 1067–1070.
8. Selective electrochemical release etching of eutectically bonded microstructures
H. Gradin, S. Braun, M. Sterner, G. Stemme and W. van der Wijngaart
The 15th IEEE Int. Conf. on Solid-state Sensors, Actuators and Microsystems
(TRANSDUCERS), Denver, USA, June 2009, pp. 743–746
LIST OF PAPERS
xi
9. Full wafer integration of shape memory microactuators using adhesive bonding
N. Sandström, S. Braun, G. Stemme and W. van der Wijngaart
The 15th IEEE Int. Conf. on Solid-state Sensors, Actuators and Microsystems
(TRANSDUCERS), Denver, USA, June 2009, pp. 845–848
xii
Wafer-level heterogeneous integration of MEMS actuators
1
1
INTRODUCTION AND STRUCTURE
1
Introduction and structure
This thesis presents research in the field of microelectromechanical systems (MEMS),
also referred to as micromachines in Japan or Micro Systems Technology (MST) in
Europe. MEMS technology uses the tools and techniques that were developed for the
Integrated Circuit (IC) industry and allows for high volume parallel production of
devices, potentially resulting in low fabrication costs per device. MEMS technology
includes components with typical sizes between 1 to 100 μm (1 μm = 0.001 mm),
which are combined to form MEMS devices such as pressure sensors, inertial sensors,
switches, pumps, valves and many more with dimensions in the mm range.
While IC devices can be considered as the ’brain’ of a microsystem, MEMS devices provide the ’arms’ and the ’eyes’ to allow the IC device to sense and manipulate
the environment. This thesis focuses on the ’arms’, i.e. actuators which typically
convert electrical energy into mechanical movement. Methods were developed to integrate actuators with silicon structures to fabricate microvalves for controlling large
gas flows and crossbar switches for automating parts of copper-wire telecommunication networks. The actuators are not integrated using the conventional monolithic
fabrication, but using concepts of heterogeneous integration. Thus, the actuators are
fabricated separately and finally bonded onto the silicon structures to functionalize.
This thesis is divided into two parts. The first part provides detailed background
information and informative references to facilitate a better understanding of the
second part, which contains the appended journal applications.
The first part contains four chapters.
In chapter 2, common MEMS actuator technologies are introduced with focus on
electrostatic and shape memory actuation, which are crucial elements of the MEMS
devices presented later on.
Chapter 3 introduces heterogeneous integration with reference to the other integration methods, which are monolithic and hybrid integration. The chapter provides
background information about heterogeneous integration concepts and technologies
including wafer-bonding methods and releasing of structures to be manipulated by
integrated actuators.
Chapter 4 introduces the concept of knife gate gas microvalves, methods to heterogeneously integrate bulk shape memory sheets and wires for actuation and the
combination of both. Other microvalve types and shape memory integration methods
are discussed. First prototypes are presented and their performance is compared to
other microvalves with respect to performance per consumed silicon footprint area.
The work is discussed and an outlook on future work is given.
Chapter 5 introduces a MEMS crossbar switch for automating parts of copper-wire
telecommunication networks. The chapter presents in detail the application, the integration of arrays of electrostatically actuated switches and a method to individually
actuate one out of the 400 switches in the array without underlying CMOS addressing
circuits. The work is discussed and an outlook on future work is given.
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Wafer-level heterogeneous integration of MEMS actuators
2
MEMS ACTUATORS
2
3
MEMS actuators
Actuators and sensors are transducers which convert one type of energy into another
one. Sensors are mostly used to measure a physical parameter and report it in form
of an electrical signal. Actuators work the other way, typically converting electrical
energy into mechanical work output. The term ’MEMS actuators’ summarizes all
actuators which typically are submillimeter sized and fabricated with MEMS technologies.
The following chapters will provide a short summary of the main MEMS actuation principles, with focus on electrostatic and shape memory actuation which are
important parts of this thesis.
2.1
Common microactuation mechanisms
MEMS technology offers a wide range of microactuators which can be classified
into electrostatic, thermal, piezoelectric, magnetic and shape memory alloy actuation methods. Table 1 and the following paragraphs present a brief overview of the
principles according to [1]. A more detailed review of all the different microactuators
and their principles would go beyond the scope of this thesis and the interested reader
is referred to literature for deeper information [1, 2, 3, 4, 5, 6, 7].
The piezoelectric actuation utilizes the coupling of mechanical deformation and
electric polarization in certain materials. When applying a mechanical stress to the
material, it generates an electrical voltage. This effect is called direct piezoelectric
effect and is used for sensing and energy harvesting applications. However, for actuation the inverse piezoeffect is deployed: by applying a voltage the material generates
mechanical movements. The stroke of piezoelectric actuators is in general very small,
however, relatively large forces can be obtained with a very precise displacement resolution. Furthermore, these actuators are fast, allowing for high cycling frequencies
and making them very feasible for repetitive actuation. An example of a commercial
application deploying the advantages of piezoelectric actuation is the smallest Piezo
LEGS® linear motor, which allows for linear travel distances limited only by the
length of the displaced element (comes with a 50 mm long element) at a speed of
20 mm
s with 10 N force and a resolution smaller than 1 nm.
For MEMS applications, the integration of piezoelectric actuators is challenging.
After the deposition the ceramic material, which most commonly is lead zirconate
titanate (PZT), must be sintered at high temperatures (> 600 °C), which limits the
4
Wafer-level heterogeneous integration of MEMS actuators
Table 1. The different principles commonly used for microactuation. The real work density
might be substantially lower.
Method
Principle
Work density
Electrostatic
Attractive force between bodies with
different electrostatic charges.
≈ 105
Piezoelectric
Shape change under an electric field (inverse
piezoeffect).
≈ 1.2·105
Thermal
Thermal expansion of single material
(includes sealed liquid) or difference in CTE
between two materials or phase change.
≈ 5·106
J
m3
Magnetic
Interaction with magnetic fields.
≈ 4·105
J
m3
Shape memory
alloy
Temperature dependent crystal phase
transformation with macroscopic shape
change. Belongs technically to thermal
actuation.
≈ 107
J
m3
J
m3
J
m3
range of compatible materials and processes. Furthermore, the ceramic shrinks during
sintering, resulting in very high mechanical stresses if the ceramic is constrained by
the substrate prior to sintering.
Magnetic actuation is based on interaction with magnetic fields which are generated by either permanent magnets or coils. The energy density of magnetic actuation
is in the same order of magnitude as electrostatic actuation. For some applications
magnetic actuation is considered as an alternative to electrostatic actuation because
of advantages such as non-existence of electric discharges, possibility of operation in
liquid and long-range forces. However, integrating magnetic field sources or low reluctance materials on-chip is a major challenge; the coils are three-dimensional structures
which are complicated to fabricate and the hard or soft magnetic materials are difficult to integrate with MEMS structures [8].
Thermal actuation is based on volume or phase change of materials upon heating
or cooling. The displacement is analog to temperature change and thereby sensitive
to environmental influences. Typically, thermal actuators provide large stroke and
large forces, however, with a rather limited displacement resolution. Heating of the
actuators can be very fast by a high applied power, however, cooling is in most cases
passive and the cooling time severely slows down the actuator and decreases the
actuation frequency.
Examples of MEMS thermal actuation approaches are the ’heatuator’ [9] using
one material, bimorph using different materials [10], thermopneumatic actuation and
shape memory alloys. The ’heatuator’ is a U-shaped lateral thermal actuator fabricated in one material, with one arm of the ’U’ considerably narrower than the other.
Current is passed through the actuator, and the higher current density in the narrower
2
MEMS ACTUATORS
5
’hot’ arm causes it to heat and expand more than the wider ’cold’ arm. The arms
are joined at the free end, which constrains the actuator tip to move laterally in an
arcing motion towards the cold arm side. The bimorph actuation scheme is based on
the difference in coefficients of thermal expansion (CTE) between two joined layers of
different materials. When changing the ambient temperature, one layer expands or
shrinks more than the other and the resulting interface stress causes bending of the
stack. The amount of bending can be controlled by choosing the appropriate CTE
combination and by the applied temperature/electrical power. The thermopneumatic
actuation scheme is based on the thermal expansion of a fluid sealed inside a cavity.
Heating causes volume expansion or even phase change, exerting a large force on the
cavity walls and causing a bending of a deformable membrane. Shape memory alloys
are materials which undergo a phase change upon temperature changes. This phase
change comes with a macroscopic shape change of the device and provides the highest
energy density of all MEMS actuators. SMA actuation is an important part of this
thesis and is presented in detail in the next subsection.
2.2
Shape Memory Alloy actuation
Technically, shape memory alloy (SMA) actuators are thermal actuators, since thermal energy triggers a crystal phase change in the material. However, since SMA
actuation is an important part of this thesis, it is presented on its own.
2.2.1
Shape Memory Effects
Structures made from Shape Memory Alloys exhibit the shape memory effect (SME),
which is the ability of certain materials to ’remember’ their initial shape after they
have been deformed. Take a spring made out of SMA and pull it. It will be easy to
deform and it will stay deformed. Now heat the spring above a specific transformation
temperature and it will rapidly recover the initial shape.
The underlying mechanism of the shape memory phenomenon is a martensite to
austenite and vice versa phase transformation. Figure 1 illustrates the different crystal
states and their connected macroscopic shapes by means of a SMA spring. In the hot
state, the SMA crystal structure thermodynamically prefers a more ordered phase and
transforms to the austenite crystal form with the macroscopic shape connected to it.
This phase is also called parent phase and it is possible to set the desired macroscopic
shape by a specific treatment involving mechanical constraining and heating. When
the spring is cooled again and there is no external force applied, it will still have
the same shape as in the hot state, however, its crystal is not in the cubic form
anymore. Instead, the layers in the material are tilted, with the tilting direction
alternating between each layer. Because of these alternated tilts, the spring remains
its shape even though the crystal form has changed. The material of the spring is
now in a macroscopically non-deformed, low-temperature phase referred to as selfaccommodated martensite [5] or ’twinned’ martensite [11], since its characteristic
alternated layers are called ’twins’. In this phase, the material features a very low yield
strength and can easily be plastically deformed after straining it above a very narrow
elastic strain range - therefore the spring can be pulled very easily. After the external
force is removed, the spring will stay deformed except for a very small elastic strain
6
Wafer-level heterogeneous integration of MEMS actuators
austenite phase
parent phase
at
in
g
he
g
in ME
at
he ay S
w
otw
co
o
lin
g
macroscopic
shape
ad
lo
y
a d it
lo tic A f
un elas >
T
rr
pe fo
g
su nly
in
o
ol
co
crystal phase
g E
in
at SM
he a y
w
eon
crystal phase
macroscopic
shape
self-accomodated martensite
twinned martensite
crystal phase
mechanical
load
macroscopic
shape
detwinned martensite
Figure 1. Illustration of the transformation processes in martensitic transformation, including all the shape memory effects.
recovery. During the deformation, the alternated layers of the twinned martensite are
moved and the macroscopic shape of the structure is changed. Now the material is
still in a martensitic phase and easily deformable. Since the alternated layers called
twins are removed, this martensitic phase is called ’detwinned’ martensite.
Figure 2 displays the hysteresis behavior of the phase transformation versus the
temperature. The transformation from martensite (cold) to austenite (hot) starts at
the austenite start temperature As and finishes at the austenite finish temperature
Af . Vice versa, the transformation from austenite (hot) to martensite (cold) starts at
the martensite start temperature Ms and finishes at the martensite finish temperature
Mf . Hence, there is no single transformation temperature. However, in practice it is
referred to a temperature T0 , which is the thermodynamic equilibrium temperature
of martensite and austenite state (T0 = MS 2+As ) [5].
There are three different shape memory effects, which are illustrated in the overview
in figure 1. In the example above, the spring must be deformed with an external force
and performs work only in one direction from detwinned martensite to austenite.
Thus, this effect is called the one-way effect. However, the material can be trained to
assume a certain shape in the cold state [12,13]. Then, the crystal transforms directly
between austenite and detwinned martensite. Hence, the material performs work in
two directions and this effect is called the two-way effect. The third effect is called
superelasticity or pseudoelasticity and is only present if the SMA is always at temperatures above the austenite finish temperature Af . Then, if an externally applied
stress overcomes a critical stress, the crystal transforms from austenite to detwinned
2
MEMS ACTUATORS
7
crystal phase
austenite
martensite
temperature T
Mf
Ms
T0
As
Af
Figure 2. Illustration of the hysteresis behavior of the martensitic transformation with the
associated temperatures: austenite start As , austenite finish Af , martensite start Ms and
martensite finish Mf . The temperature T0 is the thermodynamic equilibrium temperature
of martensite and austenite state.
martensite. When the stress is removed, the crystal immediately transforms back to
austenite. In this case, the spring in the example above will immediately, and without
extra energy supply, recover the straight shape when the deforming external force is
removed.
2.2.2
Actuation aspects of Titanium-Nickel alloys
The shape memory phenomenon was first discovered in the 1930s in brass alloys. In
1962, Buehler and his colleagues found the shape memory effect in alloys of Nickel
and Titanium [14] and in honor of their employer they named these alloys NiTiNOL
(Nickel Titanium Naval Ordinance Laboratory). Nowadays, these allows are also
known under the acronyms TiNi (Titanium-Nickel) or NiTi (Nickel-Titanium). In
this thesis, the term TiNi is used. Besides TiNi, there are a number of other materials showing the shape memory effect, such as other metallic alloys, polymers and
even bacteria [15, 16]. However, TiNi based SMA devices are dominating the market
because of several advantages over other alloy systems. TiNi alloys allow to adjust
the transformation temperature T0 over a wide range only by changing the ratio of
nickel atoms. If the alloy consists of half nickel and half titanium atoms, the transformation occurs near 100 °C. However, adding slightly more nickel atoms decreases
the transformation temperature to below 0°C. Furthermore, these alloys can be fabricated with standard metalworking techniques, they exhibit better shape memory
strain performance than other known alloys and consist of the affordable elements
Nickel and Titanium. TiNi is a biocompatible material, making it interesting especially for medical applications. All the following reflections and work presented in
this thesis are based on TiNi alloy.
SMA actuation is generally based on the one-way effect, i.e. when heated upon
deformation the structure recovers its initial shape, yet upon cooling the shape does
not change by itself. The approaches to utilize the one-way SME are usually summarized in three different categories, depending on the load which is applied upon the
SMA during shape recovery [5, 11, 17].
1. In free recovery, a deformed SMA device is not constrained by any external load
during the shape recovery and therefore the SMA does not provide any force.
8
Wafer-level heterogeneous integration of MEMS actuators
2. In constrained recovery, the shape recovery of a deformed SMA device is blocked
by an external constraint, triggering large forces from the SMA.
3. In cyclic work production, the shape recovery is constrained. However, upon
heating the SMA can overcome the external force for the shape recovery. Upon
cooling, the external force deforms the SMA again until the next temperature
cycle. The external force is called bias spring or cold-state reset.
The combination of SMA and bias spring described under cyclic work production is
the basis for SMA microactuators. The methods providing the cold-state reset can
be summarized as intrinsic and extrinsic methods [18].
Using intrinsic methods, the crystal of the material is modified to prefer a certain
cold-state crystal orientation, which results in a preferred shape of the structure in the
cold-state. An example of an intrinsic cold-state reset is the two-way shape memory
effect [19, 20], where a cold-state shape is trained into the material using long-term
cycling processes. However, compared to extrinsic cold-state reset methods, the twoway effect is very unstable [19, 20], exhibits considerably less recoverable deformation
and furthermore the required training process is difficult to integrate with a batch
fabrication process for MEMS applications [5, 21]. Therefore this method will not be
further discussed in this thesis.
Most of the SMA actuators deploy extrinsic biasing methods, where the SMA
material is coupled with an additional mechanical element. A widely used biasing
scheme, especially in MEMS applications, is coupling of the SMA with an external
biasing spring element. This topic will be addressed in detail below. Another interesting scheme is the antagonistic biasing, where two SMA elements are coupled
together. While element A remains cold and very easy to deform, element B is heated
and pulls the cold element A without requiring high forces. Then, after cooling, both
elements maintain their current shape until element A is heated, thereby deforming
the cold and soft element B. Since the SMA bias spring is very easy to deform over
relatively large strains (of course only within the elastic range), large deflections can
be obtained. Yet, in MEMS applications it is difficult to couple two SMA elements
in combination with a good thermal isolation between them.
The achievable work density of the TiNi is very much depending on the load case.
Table 2 [22] shows the different cases. Under pure tension or compression load, the
highest forces can be obtained, however, with relatively small displacements. Larger
displacements, yet with lower forces, can be obtained under torsion or bending load.
Bending load provides the lowest energy density of the three different load cases, since
only fractions of the material are used for work production.
Another important aspect to consider when designing the actuator is the fatigue
of the material. Fatigue in TiNi is usually divided into structural and functional
fatigue [23]. Structural fatigue refers to the mechanical failure of the TiNi after
cyclic loads, similar to any other engineering material. But unlike normal engineering
materials, shape memory alloys show different properties in different temperature
ranges, which also influences the fatigue characteristics.
Functional fatigue refers to a decrease in functional properties, which is the shape
2
MEMS ACTUATORS
9
Table 2. Comparison of work density and energy efficiency of TiNi wires for three different
load cases [22].
Load case
J
Work density ( kg
)
Energy efficiency (%)
Tension/Compression
Torsion
Bending
466
82
4.6
1.3
0.23
0.013
Table 3. Allowable stress and strain for a targeted amount of actuation cycles [22].
Cycles
Max.strain (%)
Max. stress (MPa)
1
102
104
> 105
8
4
2
1
500
275
140
70
recovery of the TiNi during cyclic loading. The functional fatigue is of great interest,
since it defines how many cycles the actuator can be operated depending on the stress
and strain applied to the material. Table 3 [22] shows some experimentally evaluated
benchmark numbers for TiNi wires. When straining the wire with the maximum
possible 8% or stressing it with the maximum possible 500 MPa, only one shape
recovery cycle can be obtained. To maximize the number of actuation cycles, the
applied strain should be below 1% or the applied stress should be below 70 MPa.
Thermal energy must be provided to trigger the shape recovery of the SMA. An
option is to vary the ambient temperature. However, for cyclic actuation purposes
this is rather impractical. The TiNi can be heated by electrically contacting and Joule
heating the material itself. However, the stable oxide on the TiNi makes electrical
contacting complicated. Therefore, especially for MEMS applications, the heating is
sometimes performed indirectly using a separate resistive heater, which can be contacted in a simpler way. The voltages needed to operate SMA microactuators are
compatible with microelectronics, however, high currents are necessary to provide the
relatively high power for heating of the SMA.
During operation, the TiNi transforms between two states and displays hysteresis behavior as described earlier. Because of the two stable states, the SMA is very
suitable for applications that require digital mode operation of the actuator. For
such applications, the hysteresis behavior is potentially of advantage; the thermal
energy necessary to maintain the austenite state is lower than the initial austenite
start temperature As , which defines the stable state very well even for an unstable
thermal energy supply. For applications requiring precise analog-like control over the
displacement of the actuator, there are several controlling solutions. One method is
the model-based loop, which is based on extensive modeling of the materials behavior
to reduce or compensate the hysteresis effect. However, the necessary material param-
10
Wafer-level heterogeneous integration of MEMS actuators
pulled maintains current
out
state
d
k
Fs
moveable plate, Area A
gradual deflection
d0
pull-in
2d
3 0
d0
d
Fel
V
pulled
in
pull-out
fixed plate
Vpull-out
(b)
(a)
Vpull-in
V
(c)
Figure 3. Diagram illustrating the operational behavior of an electrostatic actuator: (a)
and (b) Parallel-plate capacitor model showing the principle of electrostatic actuation with
and without applied voltage, respectively; (c) Diagram illustrating the hysteresis behavior
of an electrostatic actuator, showing the typical pull-in and pull-out characteristic.
eters must be experimentally evaluated. Another method, the feedback-loop, relies on
sensing of either position [24], temperature [25] or electrical resistance [26] of the SMA
structure as input. Both position and temperature sensing require additional devices
and especially the temperature sensing is impractical due to temperature disturbances
in an open environment. The control based on electrical resistance sensing is very interesting since it utilizes the smart material capabilities of the SMA. Yet another
interesting approach is to keep the digital mode operation with all its advantages, but
segmenting the SMA element to quantify the deflection [27].
This overview is far from being complete, therefore the interested reader is referred
to [28] for more information on this topic.
2.3
Electrostatic actuation
The electrostatic actuation principle relies on the attraction force between bodies having different electrostatic potential caused by a charge inbalance. A simple example
of an electrostatic actuator is a parallel-plate capacitor, as illustrated in Figure 3a,
with one fixed plate and the other plate suspended by a mechanical spring with a
spring constant k at an initial distance d0 . Applying a voltage V between the two
plates results in a vertically attractive electrostatic force, which pulls the moveable
plate towards the fixed plate (Figure 3b). Using this simplified model and neglecting
fringe-fields, the electrostatic force Fel between the plates can be calculated as [29]
Fel =
A
1
ε0 r 2 V 2
2
d
(1)
with ε0 the permittivity of free space, r the effective relative permeability, A the
overlap area of the two plates and d the distance between the two plates.
This formula is the basic formula for all electrostatic actuators and shows that the
electrostatic force grows quadratically with decreasing distance between the plates,
which makes electrostatic actuation very interesting for MEMS applications with very
small gap distances below tens of micrometers.
2
MEMS ACTUATORS
2.3.1
11
Parallel-plate, comb-drive and curved-electrode actuators
For electrostatic actuators based on the parallel-plate concept there are several issues
to consider. The electrostatic force Fel is counteracted by the mechanical spring force
Fs , which is calculated as:
Fs = −k(d0 − d)
(2)
For practical designs with a low actuation voltage, small electrode area and a
sufficiently stiff mechanical spring, the initial plate distance d0 must be small, which
results in small travel distances d (strokes) of typically a few micrometers for the
moveable plate. Furthermore, the range in which the stroke of the moveable plate
can be controlled is limited. Figure 3c illustrates the deflection of the moveable plate
during a full operation cycle. With increasing actuation voltage, the gap between the
plates gradually decreases and the two forces Fel and Fs will settle in an equilibrium.
However, with decreasing d, the electrostatic force grows quadratically, whereas the
counteracting spring force only grows linearly. At distances smaller than the critical
distance d = 23 d0 , there no longer exists an equilibrium between the forces and the
moveable plate snaps down to the fixed plate. To avoid an electrical short-circuit after
snap-down, there must be an electrical isolation layer or at least ’dimples’ (distance
holders) between the plates. The critical distance is independent of the geometrical
parameters of the actuator and the voltage at which the plate snaps down is called
the pull-in voltage, or Vpull−in (figure 3c). After the pull-in, the gap d is drastically
minimized and therefore, when reducing the applied voltage after pull-in, the electrostatic force remains larger until a force equilibrium is reached again. When further
reducing the applied voltage, Fs overcomes Fel and the moveable plate is pulled out.
Accordingly, the voltage at which the pull-out occurs is called Vpull−out (figure 3c).
Some applications require an analog behavior of the actuator and there are efforts
to extend the limited analog controllable stroke of parallel-plate actuators [30,31,32].
However, there are also many applications demanding a digital mode operation of
the actuator, such as electrical and optical switches which alternate between the
ON and the OFF state. In these applications, the hysteresis behavior is actually of
advantage; the voltage necessary to maintain the pull-in state is lower than the initial
actuation/pull-in voltage, which defines the switching state very well even at unstable
control voltages.
In contrast to the parallel-plate actuator, the ideal comb-drive actuator shows
no pull-in and hysteresis behavior since the plates are not moving perpendicularly,
but parallel to each other and thereby keep the plate distance d constant during the
operation. A second fixed plate is added and the moveable plate is interdigitated
between the two fixed plates with an initial lateral overlap x0 . Figure 4 illustrates the
actuator, which is called ’comb-drive’ since the interdigitated finger-like structures
look like the teeth on a comb.
Applying an actuation voltage results in several forces, as illustrated in figure 4b.
The two vertical force components Fel,y keep the moveable finger centered between
the stationary fingers and the lateral force component Fel,x pulls the moveable finger towards the fixed fingers and counteracts the lateral bias spring with the spring
12
Wafer-level heterogeneous integration of MEMS actuators
fixed plate, thickness t
x0
Fel,y
x
d
moveable plate, thickness t
V
k
Fel,x
Fs
d
Fel,y
fixed plate, thickness t
(a)
(b)
Figure 4. Diagram illustrating the operational behavior of an electrostatic comb-drive
actuator, without (a) and with (b) applied actuation voltage.
constant k.
The lateral force Fel,x is independent of the plate overlap and remains constant
with increasing plate overlap. The distance between the capacitor plates also remains
constant, which allows for large analog controllable stroke, only limited by the elastic
range of the bias spring.
However, the large stroke comes at a cost. The fingers are usually fabricated by
vertical etching into the silicon substrate using deep reactive ion etching (DRIE). As
for all electrostatic actuators, the distance d between the fingers should be as small as
possible and the electrode area A as big as possible. Consequently, high aspect ratio
processes are necessary to produce structures with a minimal distance in between.
Since the aspect ratio and the resulting initial distance is limited, the only way to
increase the electrode area and the force is the massive parallelization of comb structures, at the cost of silicon footprint.
Both large stroke and large force are provided by curved-electrode actuators, which
utilize a flexible beam opposite a fixed electrode. Figure 5 illustrates the principle.
One end of the flexible beam is clamped with a very short distance to the fixed
electrode. One of the two electrodes is a curved electrode, shaped in a way that
the electrode distance is gradually increasing from the clamped end to the free end.
Upon applying an actuation voltage, the narrow gap at the clamped end results in
large forces and the flexible beam is pulled in. As for the parallel-plate actuator,
electrical isolation between the beams is necessary to avoid an electrical shortcircuit.
The point of pull-in is moving along the fixed electrode in a zipper-like way and
therefore these actuators are also referred to as ’zipper-actuators’. Another name
is ’touch-mode’ actuator, since these actuators utilize the pull-in and the plates are
touching each other only separated by a thin electrical isolation layer or stoppers.
The combination of small plate distance at the clamped end, the touching mode with
very thin gaps between the electrodes and the large distance at the free end results
in large forces and a large stroke, making this actuation scheme very interesting for
MEMS applications.
For the most common zipper actuators, the moveable part is moving either laterally or vertically, as illustrated in figure 5 [33]. Besides the orientation of the
actuation, the two configurations also differ in the arrangement of fixed and moveable
plate as well as in their fabrication. In the lateral zipper approach, the fixed electrode
2
MEMS ACTUATORS
13
d0
moveable,
straight
electrode
fixed,
curved
electrode
moveable, curved
electrode
d0
fixed, straight
electrode
silicon
(a)
(b)
Figure 5. The two most common fashions of zipper actuators: (a) lateral zipper and
(b) vertical zipper. The figure is modified from [33].
is curved and the moveable electrode is a straight cantilever. The fabrication is fairly
simple with one photolithographical mask, vertical etching into the device layer of
a SOI wafer and sacrificially underetching the buried oxide to release the moveable
beam. However, as for the comb-drive actuator, the initial gap and the electrode
area are limited by the aspect ratio of the fabrication process. In the vertical zipper approach (illustrated in figure 5b and 6a), the fixed electrode is straight and the
moveable electrode is curved, typically fabricated using surface micromachining. The
curvature of the moveable electrode results of a controlled, fabrication process related
stress gradient and because of the curvature, these kind of actuators are also called
’curled actuators’. In contrast to the lateral approach, the initial gap at the clamped
end can be very narrow by utilizing a thin sacrificial layer and the electrode area can
be very large, resulting in large forces at relatively low actuation voltages. However,
the stress gradient in the moveable electrode and the resulting spring tension counteracting the electrostatic force is difficult to control. Furthermore, stiction could occur
between the large area electrodes in close contact.
2.3.2
S-shaped film actuators
The stress gradient in the bending electrodes of standard vertical zipper actuators
is difficult to control. A large stress gradient results in a spring with a high pretensioning, counteracting the electrostatic force and resulting in larger actuation voltages to pull in the electrode. A thin and soft membrane would decrease the necessary
actuation voltages, however, a weak spring cannot provide a reliable pull-out and
suspension of the electrode.
A solution allowing for a thinner and softer membrane is the incorporation of a
second fixed electrode at the free end of the membrane, providing a second zipper
actuator, as illustrated in figure 6b. The resulting double-zipper actuator provides
active actuation of the film in both directions, allowing for a very flexible membrane
with a low stress gradient and thereby potentially reducing the actuation voltage yet
still allowing for a large stroke. A MEMS concept of such an actuator is illustrated
in figure 6b. First, a single zipper is fabricated with a thin and flexible membrane,
14
Wafer-level heterogeneous integration of MEMS actuators
pre-stressed
flexible electrode
d0
touch-mode, pull-in ’zips’
along fixed electrode
V
V
fixed electrode
(a) single ’zipper’ actuator
2nd fixed electrode
two ’zippers’, d0 defined by thin electrical
isolation layer between the electrodes
d0
fixed electrode
V
d0
(b) double ’zipper’ or S-shaped film actuator
Figure 6. Illustration of (a) single vertical zipper actuator and (b) double vertical zipper
or S-shaped film actuator.
yet still with sufficient bending of the free end. Then, the second electrode is added
from the top and pushes the membrane in contact with both electrodes and creating
the characteristic S-shape of the membrane which inspired the name of the S-shaped
actuator.
To allow the membrane to move up and down, the two fixed electrodes must be
kept at a distance to each other with an intermediate spacer. The thickness of this
spacer allows to tune the distance the membrane can move up and down between the
two electrodes, which defines the stroke of the actuator.
In summary, this concept comes with a set of advantages. The touch-mode actuation, with very small initial gaps in both directions, in combination with a thin and
flexible membrane potentially results in very low actuation voltages. The stroke of
the actuator is basically only limited by the tuneable spacing between the two fixed
electrodes.
The S-shaped actuator was shown in 1997 for a gas valve with dimensions in the
millimeter range [34]. Another work [35, 36, 37] utilized the assembly concept of the
S-shaped actuators to fabricate an RF MEMS switch by fabricating a single zipper actuator with metal contacts on one substrate and combining it with a second substrate,
which contained the second fixed electrode and signal lines to be interconnected. In
this work, the spacing between the substrates was provided by a polymer ring, which
also encapsulated and packaged the switch.
3
HETEROGENEOUS INTEGRATION
3
15
Heterogeneous integration
Heterogeneous integration evolved from monolithic and hybrid integration and refers
to the wafer-level integration of different materials, technologies or devices. In monolithic integration a device is fabricated in one piece while in hybrid integration a device
is fabricated by interconnecting several separate pieces. The following sections introduce the different integration methods, followed by technical background including
methods for wafer-to-wafer bonding, vertical electrical interconnection and releasing
of structures for actuation.
3.1
Introduction
The following sections introduce the concepts of monolithic, hybrid and heterogeneous
integration. Heterogeneous integration is of high interest for the integration of MEMS
and IC and therefore the integration technologies are introduced by means of the
specific example of integrating MEMS materials onto IC circuits.
3.1.1
Monolithic and hybrid integration
In monolithic integration, devices are fabricated from one substrate (monolithic =
made from one piece). All the processing is typically performed on wafer-level and
after the fabrication the wafer is diced into discrete devices (figure 7a), which are
ready for further application.
As an example, MEMS and IC are monolithically integrated by combining and
customizing the MEMS and IC manufacturing processes. The main technical advantage of monolithic integration of MEMS and IC is the high integration density;
electrical interconnections between MEMS and IC are very short, reducing electrical
noise and allowing for the handling of small signals. However, monolithic integration
of MEMS and IC is relatively complicated [38, 39, 40, 41, 42], since MEMS technology
can require IC incompatible material deposition processes and/or temperatures above
450 °C, which is not allowed for the IC components.
A solution to avoid these problems is the hybrid integration (hybrid = combination
of different parts), where the devices are fabricated on separate substrates, which are
then diced into single chips and combined with each other on chip level (figure 7b).
As an example, MEMS and IC are hybrid integrated by fabricating on separate
substrates, which are then diced into single MEMS and IC chips. Conventionally,
16
Wafer-level heterogeneous integration of MEMS actuators
MEMS+IC
substrate
dicing
Device 1
Device 2
Device 3
(a) monolithic integration
MEMS
substrate
IC
dicing
MEMS 1
substrate
dicing
MEMS 2
MEMS 3
IC 1
IC 2
IC 3
pick-and-place
assembly
wirebond
adhesive
carrier substrate
Device 1
carrier substrate
Device 2
carrier substrate
Device 3
(b) hybrid integration
MEMS
IC
target
substrate
source
substrate
bonding
removing source substrate
dicing
Device 1
Device 2
Device 3
(c) heterogeneous integration
Figure 7. Simplified schematic illustrations of the different methods for integrating MEMS
with IC: (a) monolithic integration, (b) hybrid integration and (c) heterogeneous integration.
3
HETEROGENEOUS INTEGRATION
MEMS
17
ASIC
wirebonds
(a)
(b)
(c)
(d)
Figure 8. Example of wire bonding based hybrid integration of a MEMS inertial sensor
with an ASIC for automotive applications [49]. (a Leadframe. (b) The MEMS sensor (top)
and the ASIC (bottom) are adhesively mounted onto the leadframe. Using wirebonding, the
MEMS is electrically connected to the ASIC and the ASIC is connected to the leadframe.
(c) Packaged by plastic molding. (d) The leadframe pins are punched free and shaped,
resulting in a chip ready for integration in a larger system.
these chips are glued beside each other on a carrier substrate and electrically interconnected by wire-bonding (see example in figure 8) or by connections integrated in
the substrate (Multi Chip Modules [43]). Alternatively, they are stacked on top of
each other [43, 44] using through-substrate-vias (TSV) [45, 46, 47, 48] for vertical electrical interconnection. The main technical advantage is the uncomplicated integration
of different technologies, materials or devices. However, there are applications where
hybrid integration is not feasible due to cost-efficiency reasons, limited integration
density and parasitic signal noise from the long electrical interconnections.
Heterogeneous integration allows to combine the two approaches and is presented
in the next section.
3.1.2
Heterogeneous integration
Typically, heterogeneous integration is utilized for integrating materials or technologies which otherwise are very difficult to combine or even incompatible with each
other. However, heterogeneous integration also allows to divide the fabrication of
MEMS devices into several separate sub-structures, which are optimized for a certain
aspect and finally combined to one device. Both aspects are included in this thesis.
Chapter 4 addresses the integration of an incompatible actuator material with silicon
structures and chapter 5 addresses the separate fabrication of MEMS actuators arrays
and their integration to functionalize another MEMS device.
Heterogeneous integration follows the same basic concept as hybrid integration.
The devices to be integrated, or parts of them, are fabricated separately. However, in
contrast to hybrid integration, the two different substrates are integrated on waferlevel by bonding them on top of each other, followed by removing the substrate of
the integrated device and dicing into single chips (figure 7c). This wafer-level hybrid integration method allows to combine the advantages of hybrid and monolithic
integration such as separate fabrication and wafer-level processing including high integration density and short electrical interconnections between the devices.
An example to demonstrate all the benefits of heterogeneous integration is the replacement of the mirror material of micromirror arrays from aluminum to monocrys-
18
Wafer-level heterogeneous integration of MEMS actuators
fabricate actuation
electrodes
IC substrate
’dummy IC’ substrate
spin-on and pattern
sacrificial layer
fabricate actuation
electrodes
spin-on and pattern
sacrificial layer
SOI - wafer
deposit mirror material
by sputtering
aluminum
deposit mirror material
by bonding SOI –
wafer and removing
handle wafer and
buried oxide
pattern mirror material
to form micromirrors
pattern mirror material
to form micromirrors
electroplate vias for
electrical interconnection and
mechanical clamping
remove sacrifial layer
(a) monolithic
remove sacrifial layer
(b) heterogeneous
Figure 9. Simplified illustrations of the fabrication of micromirrors: (a) monolithically
fabricated with aluminum mirrors [50] and (b) heterogeneous integration of a silicon layer
for the mirrors [51].
talline silicon. The famous digital micromirror device (DMD) for projectors from
Texas Instruments [52] features an array of up to 2048×1152 micromirrors and each
of these mirrors must individually addressable, which for the DMD is performed using
a dedicated IC circuit. Using hybrid integration of the mirrors and the IC would not
be feasible since more than one million interconnection wires would be necessary to
control each mirror. Therefore, the mirrors are monolithically integrated on top of
the IC by sputtering and patterning aluminum as mirror material. Figure 9a [50]
shows a simplified example of such a process. Each mirrors control electrode is addressed by a memory cell directly underneath, which eliminates the need to route
individual control wires underneath the array. However, after repeated or prolonged
mirror actuation, the aluminum mirrors display hysteresis and memory effects, which
can be problematic for applications requiring analog mirror deflections. These issues
can be eliminated by utilizing monocrystalline silicon for the mirrors. Furthermore,
the achievable optical quality, the surface roughness and the uniformity of monocrystalline silicon surfaces is superior compared to most other surfaces. However, since the
process temperatures for the material deposition onto IC is limited to about 450 °C
to avoid damage to the electronic circuits, many high-performance MEMS materials
such as monocrystalline semiconductors cannot be monolithically integrated.
A solution is the heterogeneous integration of monocrystalline silicon mirrors onto
the IC driving circuitry by transferring the thin silicon device layer of a SOI-wafer
using a IC compatible transfer bonding process. The involved temperatures are always
below 450 °C. Figure 9b shows a simplified example of such a process [53,54,55,56,51]
3
HETEROGENEOUS INTEGRATION
19
and the analogy to the monolithic fabrication of micromirror arrays.
Heterogeneous integration was enabled with the advent of wafer bonding technologies and is an important technology for the ’More than Moore’ trend to integrate
non-IC functions onto IC devices in order to increase their capabilities beyond Moores
Law [57]. More information about the technology behind heterogeneous integration
is presented in the following section.
3.2
3.2.1
Heterogeneous integration concepts and technologies
Transfer/direct and wafer-to-wafer/chip-to-wafer integration
The illustration of heterogeneous integration in figure 7 shows the transfer integration
approach, which allows to transfer layers of one substrate to another substrate. These
layers can be closed layers or devices which are fabricated in the same layer. The layer
to be integrated is first fabricated on a temporary carrier substrate, which is the source
substrate. The source substrate is then bonded upside down onto the target substrate,
followed by removal of the source substrate. As a result, the source layer to be
integrated is bonded upside down onto the target substrate. If the element should not
be integrated upside down, it can be bonded to another intermediate substrate before
it is transferred to the final target substrate. An example for transfer integration
is the integration of the thin device layer of a SOI wafer onto CMOS substrate for
micromirror arrays.
In contrast to transfer integration, in direct integration the substrates are bonded
directly on top of each other without intermediate carriers or removing substrates.
The substrates can be bonded with the top or the bottom side facing each other,
depending on the application. Direct integration allows for wafer-scale encapsulation/packaging of devices as illustrated in figure 10. The source substrate is bonded
with the top side onto the top side of the target substrate. The bonding layer is
thick and patterned, forming a ring around the devices and encapsulating/packaging
the devices after the bonding. Encapsulating MEMS structures by bonding plain
wafers with recesses on top of the target wafer is a common wafer-level packaging
approach [58]. However, in contrast to only one substrate containing devices, heterogeneous integration allows for both of the substrates containing functional structures.
The integration approaches introduced above allow for integrating wafers to wafers,
which is typically cost-efficient if the devices on both substrates feature similar footprint areas or if the total cost per footprint of the substrate with the smaller devices
is much lower as compared to the final total cost of the final devices. Figure 11 illustrates this issue: if the device to be integrated features a much smaller footprint
area than the device on the target substrate, a lot of expensive substrate material is
be wasted.
The conventional alternative to integrate devices with largely different areas would
be the hybrid integration. However, this requires robotic pick-and-place of the components, which is a serial process and especially for high volume production may not
be cost-effective. Alternatives based on heterogeneous integration are chip to wafer
integration methods, where the source substrate with the source devices is diced into
single chips, which are which are bonded onto the target devices on the un-diced
20
Wafer-level heterogeneous integration of MEMS actuators
target devices
bonding layer
devices to be
integrated
encapsulated/packaged
devices
rings
cross-section
top view
Figure 10. Schematic illustration of direct integration which results in wafer-level encapsulation/packaging of the integrated devices, based on a thick bonding layer which is patterned
to form rings around the devices.
target wafer. These methods address the cost issue associated with largely different
footprint areas and three examples of such techniques are described below.
One method is mixing pick-and-place and wafer-level transfer integration. The
source substrate is diced into single chips, which are pick-and-place bonded onto the
target devices on the target substrate. This approach has been developed by IMECGhent University for the integration of optical chips onto IC substrates [59], using an
intermediate substrate to bond the chips with the correct side to the target substrate.
Another approach utilizes self-alignment methods, which are part of self-assembly
methods [60, 61, 62, 63]. The smaller devices are densely fabricated on a source substrate which is diced into single chips. These single chips are placed on an assembly
wafer, without any orientation or order and due to previous manipulations of the assembly wafer the chips orient themselves along defined patterns with defined pitches.
Induced vibration or evaporating liquid helps to overcome the friction between the
chips and the surface of the assembly wafer. After the chips have oriented and assembled themselves on the assembly wafer, they can be transferred onto the target wafer.
Alternatively, the chips are self-assembled directly on the target wafer. Similar methods have been used to transfer monocrystalline silicon onto surface micromachined
electrostatic actuators to fabricate micromirrors [64]. Furthermore, there is a report
using self-assembly for the integration of MEMS (actuators) with IC [65].
A third method is the ’selective transfer technology for microdevice distribution’
presented by IBM [66]. Their concept involves the dense fabrication of the smaller
devices on a source wafer. This source wafer is aligned to the target wafer in a
way, that one small source device is aligned to the larger target device on the target
wafer. Then, the source device is released from the source wafer and transferred
to the target device. By adapting the pitch of the smaller elements and the target
devices, several smaller elements can be transferred in one transfer step. Using this
technology, one source wafer can populate a number of target wafers and thereby the
3
HETEROGENEOUS INTEGRATION
21
”wasted”
substrate area
source devices
target devices
(a)
(b)
Figure 11. Cost-effectiveness of wafer-to-wafer integration: (a) The integration is costeffective because the footprint of the transferred device is similar to that of the target device.
(b) The wafer-integration is no longer cost-effective due to the large footprint differences.
cost of the transferred wafer is distributed over the number of target wafers. IBM
showed this method for the distribution of their AFM-cantilevers and, together with
FZK Karlsruhe, they showed this method for integrating bulk TiNi actuators onto
polymer microvalves [67].
3.2.2
Electrical interconnection
One of the advantages of heterogeneous integration is that the electrical interconnections between the integrated devices can be made very short and dense, reducing
parasitic influences. There are applications, where electrical interconnection between
the integrated devices is not necessary (such as the integration of SMA, which is
described in later sections). Yet, if electrical interconnections are necessary (as for
example in the micromirror arrays), these interconnections are in most cases electrically conductive vertical vias. The methods for providing electrical vias can be
distinguished in the via first and the via last approach. In the via first approach,
all vertical electrical interconnections are fabricated prior to integration and during
bonding the devices are electrically interconnected as illustrated in figure 12a. One
of the advantages of this approach is that the two substrates can be fabricated completely separately, are then electrically interconnected during the integration and no
further post-integration processing is required for the electrical contacting. However,
this method requires a careful alignment of the two substrates, which limits the size
reduction of the vias.
In the via last approach, only the electrical contact pads on the target substrate are
fabricated prior to integration. After the bonding, vias are etched into the integrated
substrate and filled with conductive material, as illustrated in figure 12b. In this
approach, no precise wafer-to-wafer alignment is necessary and the vias can be made
considerably smaller than in the via first approach. However, the integrated substrate
22
Wafer-level heterogeneous integration of MEMS actuators
electrical contacts
substrate removed
preparing electrical contacts
the devices are electrically
prior to integration
interconnected during the bonding
(a) via first
electrical contacts
preparing electrical contacts
only on target devices
substrate removed
bonding of the substrates
vias
etching of vias in the integrated
devices and filling them with
conductive material
(b) via last
Figure 12. Illustration of the two approaches for electrical via fabrication: (a) the via
first approach, where the connections are fabricated prior to integration and (b) the via last
approach, where the connections are fabricated after integration.
must be processed after the bonding. As an example for the via last approach, the
silicon micromirrors in the mirror arrays (figure 9b) are electrically connected by vias
etched through the silicon and the sacrificial layer to the electrical contact pads on the
IC substrate. These vias are filled with metal to connect the mirrors to the driving
circuitry and allow for electrostatic actuation of the mirrors.
3.2.3
Wafer-bonding techniques
A key technology of heterogeneous integration is the bonding of wafers to each other.
The following brief descriptions are based on wafer bonding review papers [68,69]. In
principle, all the mentioned bonding methods are suitable for heterogeneous integration.
In solder bonding [70, 71, 72], layers of metal or metal-alloy based solders are
used to bond two wafers. The metal layers are usually deposited on both wafers,
which are joined and heated to the melting temperature of the solder. The solder
reflows and wets both wafer surfaces, causing intimate contact and bonding of the
surfaces. Example solder materials are lead-tin (Pb–Sn), gold-tin (Au–Sn) and tincopper (Sn–Cu) solders. Oxides at the metal surfaces can result in poor bonding and
therefore most solder bonding processes use flux to remove the oxides. To some extent,
solder bonding tolerates particles and structures at the wafer surfaces. The method
provides hermetic bonding/packaging. Furthermore, it allows for combined bonding
and vertical electrical interconnection, making it very interesting for heterogeneous
integration.
Eutectic bonding [73,74,75,76,77] is a variation of solder bonding, allowing to join
3
HETEROGENEOUS INTEGRATION
23
two wafers with dissimilar surface materials which form a eutectic mixture at temperatures much lower than their melting temperatures. The most common material
combination is silicon (Si) and gold (Au) with a eutectic temperature of 363 °C. Eutectic bonding can result in strong and hermetic bonds at relatively low temperatures
and is therefore often used for the hermetic sealing of micromachined transducers.
Furthermore, the method is interesting for heterogeneous integration because it allows for vertical electrical interconnection.
In adhesive bonding [69, 78, 79, 80], an intermediate adhesive layer creates a bond
between two surfaces. Most commonly, a polymer adhesive is applied and the wafers
are pressed together. Then, the polymer adhesive is hard-cured, typically by exposing to heat or ultraviolet (UV) light. The main advantages include the relatively low
bonding temperatures between room temperature and 450 °C (depending on the polymer material), the insensitivity (to some extent) to the topology or particles on the
wafer surfaces, the compatibility with standard complementary metal-oxide (CMOS)
semiconductor wafers and the ability to join practically any wafer materials. While
adhesive wafer bonding is a comparably simple, robust and low-cost process, concerns
such as limited temperature stability and limited data about the longterm stability
of many polymer adhesives in demanding environments need to be considered. Also,
adhesive wafer bonding does not provide hermetically sealed bonds towards gasses
and moisture. The method does not provide electrical interconnection.
In direct or fusion bonding [81,82,83], two wafers are contacted without significant
pressure, electrical fields or intermediate layers. For reliable bonding, this method
requires very flat and very clean wafer surfaces, room temperature contacting of the
wafers and an annealing step (typically between 600 and 1200 °C) to increase the
bond strength. This method results in strong and hermetic bonds and is therefore
of interest if the integration method should also provide hermetic packaging. The
method does not provide electrical interconnection.
Anodic or field assisted bonding [84,85] is based on joining an electron conducting
material such as silicon and a material with ion conductivity such as alkali-containing
glass. Heating to temperatures of 180–500 °C mobilizes the ions and an applied
voltage of 200–1500 V creates a large electric field that pulls the wafer surfaces into
intimate contact and fuses them together. Anodic bonding is more tolerant to surface
roughness than direct bonding and usually leads to strong and hermetic bonds. The
method is interesting if hermetic packaging is required, however, the large voltages
might damage IC devices on the substrates. The method does not provide electrical
interconnection.
Thermocompression bonding, metal-to-metal direct bonding, and ultrasonic bonding [86, 87, 88, 89] are related bonding schemes in which two surfaces are pressed
together and heated. Typically at least one of the surfaces consists of a metal. The
surfaces plastically deform and fuse together. Instead of heating, the energy can also
be supplied by ultrasonic energy (ultrasonic bonding), with the advantage of breaking through native oxides, particles and surface nonuniformities at the bond interface.
Common bonding surface materials are gold to gold, copper to copper, aluminum to
gold, and aluminum to glass. The disadvantage of thermocompression and ultrasonic
bonding is that large net forces are required when bonding larger wafer areas. Thus,
thermocompression bonding, metal-to-metal direct bonding and ultrasonic bonding
24
Wafer-level heterogeneous integration of MEMS actuators
are mainly used in wire bonding schemes and in bump bonding schemes. However,
these methods provide hermetic bonding/packaging and the possibility for vertical
electrical interconnection, making it very interesting for heterogeneous integration.
In low-temperature melting glass bonding [90] an inorganic low-temperature melting glass or glass frit layer forms the intermediate bonding material and is deposited
on one or both of the wafers. The wafers are joined and heated, causing the glass
to deform or reflow and bonding the wafers. Two different types of glasses are available; devitrifying glasses, of which the melting point is permanently increased after
the curing and vitreous glasses, which always melt at the same temperature. This
method allows to hermetically bond various wafer materials at relatively low bonding temperatures and tolerates to some extent particles and structures at the wafer
surfaces. The method does not provide electrical interconnection.
3.2.4
Releasing structures for actuation
When integrating actuators there are some more issues to consider beside wafer bonding and vertical electrical interconnection. Actuators imply moving structures which
must be detached from their underlying bonding layer to allow their movement. For
structures fabricated using wafer-to-wafer bonding, the techniques to detach them
from their underlying substrate can be summarized in two approaches. The first
method is the localized bonding of areas to be affixed while avoiding the bonding of
the structures to be detached. The second method is a bond-and-release approach,
in which all structures are bonded to the substrate, followed by removing the bond
interface material underneath the structures to be detached.
Localized bonding (illustrated in figure 13a) between two substrates can be obtained using two different principles. The first principle is to modify the interface
material prior to bonding, defining bonding and non-bonding areas. Examples of
patterned bond interface layers include adhesive layers applied only on areas where
bonding is desired [80] and bond blocking layers such as gold or platinum defining local non-bonding areas in anodic bonding [91]. The second localized bonding principle
is to use heat triggered bonding methods and to localize the heat to the desired areas
of the bond interface. Examples of this approach include integrated heaters for both
localized eutectic and silicon fusion bonding [92], localized soldering using inductive
heating [93] as well as local heating using lasers [94].
In localized bonding, the non-bonded parts are either fallout-structures or they
must remain mechanically connected to the bonded parts by mechanical supports to
prevent them from falling out during the remaining process steps. The removal of
mechanical support structures through dicing or through controlled fracture has been
shown [95, 96]. However, such break-away structures limit the design freedom and
potentially increase the footprint area of the MEMS device. Furthermore, the moving
structures could be damaged while removing the support structures.
The most common technique for releasing bonded structures is the bond-andrelease approach based on sacrificial underetching (illustrated in figure 13b). This
technique requires the fabrication of the structures on top of a ’sacrificial’ layer,
which can be etched with a high selectivity. This approach is common in surface
3
HETEROGENEOUS INTEGRATION
to remain bonded
25
to be detached
not
bonded
bonded
(a) localized bonding of the structures to remain bonded
to be detached,
with etch holes
to remain bonded,
without etch holes
underetch distance w
w/2
bonding layer
(b) sacrificial underetching with etch holes
to be detached
to remain bonded
bonding layer
bonding layer is
locally dissolved
bonding layer is
not attacked
(c) localized removal of the bonding layer
Figure 13. Illustration of the different methods to detach structures from the substrate:
(a) localized bonding of areas to be affixed while avoiding the bonding of the structures to be
detached; (b) sacrificial underetching with etch holes; (c) localized removal of the bonding
layer only underneath the structure to be released.
micromachining [97], where the layers are stacked upon each other. In the crossbar switches presented later in this thesis, the actuators are fabricated using surface
micromachining and released prior to integration with the target substrate. A wide
variety of sacrificial materials have been demonstrated, such as silicon dioxide [98],
polymers [99, 100] and metals [101]. In structures fabricated by wafer-bonding, sacrificial underetching is based on wafer-bonding methods with intermediate bonding
layers that can be sacrificially etched with a high selectivity to release the attached
structures. Examples of such intermediate bonding materials are silicon dioxide [102]
and polymers [69, 103]. It should be noted that processes using the buried oxide
in silicon-on-insulator (SOI) wafers as sacrificial layer can be considered as such a
bond-and-release technology.
A challenge in sacrificial underetching is to provide the selectivity between structures to be detached and structures to remain bonded. If the whole sacrificial layer
is removed, there is no selectivity and all structures are detached. One solution for
this issue is to affix the structures not to be detached via a second clamping material,
which is not attacked when etching the sacrificial layer. An example for this strategy
are again the silicon micromirror arrays: the silicon is transfer integrated using adhesive polymer bonding onto a polymer sacrificial layer. Then, metal vias which provide
both electrical and mechanical interconnection from the silicon to the IC substrate
are fabricated (see figure 9b). Finally, all polymer is removed but the mirrors are still
clamped by the metal vias, which are not attacked during the polymer ashing.
Another approach which does not require a second clamping, is to provide the
26
Wafer-level heterogeneous integration of MEMS actuators
selectivity by integrating vertical etch holes in the structures to be detached but not
in the substrates to remain bonded. These etch holes drastically minimize the distance
to underetch as compared to structures without etch holes. Thus, the structures with
etch holes are detached while the structures without etch holes are only partially
underetched at their edges (see figure 13b).
Etch holes are commonly included in sacrificial underetching to minimize the time
the whole substrate is exposed to the etchant or the etch process environment, which
may result in device destruction if other materials than the sacrificial layer are attacked. Yet, beside the advantages there are also some challenges to consider. Etch
holes potentially decrease the mechanical stability and the performance of the structures to be released. Furthermore, the fabrication of such etch holes is feasible only
for structures consisting of thin layers, such as the thin silicon layers of micromirror
arrays. If the structures are hundreds of micrometers thick, etch holes are difficult
to fabricate with the required aspect ratio. Even if such deep and thin vertical etch
holes could be fabricated, the etching of the underlying layers would presumably be
very slow due to poor mass transport and depletion of the etchant in the etch holes.
An alternative, which allows for the area-independent release of bonded structures without additional etch holes and harsh chemical environments, is the localized
removal of the intermediate bonding layers (illustrated in figure 13c). An example
of localized removal is the localized laser ablation of a polymer bonding layer [66].
Another example is based on the electrochemical etching of metal sacrificial layers in
a neutral salt solution. This principle has been shown for detaching surface micromachined structures [104], which were deposited onto aluminum as sacrificial layer. In
this thesis, this method was adapted to release eutectically bonded silicon microstructures to allow for their manipulation by heterogeneously integrated SMA actuators.
Further information about this release etch technique can be found in the attached
paper 4.
4
KNIFE GATE MICROVALVES WITH BULK SMA MICROACTUATORS
4
27
Knife gate microvalves with bulk SMA microactuators
This chapter introduces methods for wafer-level integrating the shape memory alloy
Titanium-Nickel (TiNi) and their combination with knife gate microvalves which are
a new tool for controlling large gas flows. First experiments proof the great promise
of the knife gate valve concept, however, the thermal bimorph actuator of the first
test devices proved not robust enough and bulk TiNi material was identified as a
promising replacement. There were no reported methods to wafer-level integrate
bulk TiNi actuators with silicon structures and, thus, no methods for combining
bulk TiNi with the knife gate microvalve concept. TiNi is difficult to integrate with
conventional methods and therefore large parts of this thesis deal with developing
methods for heterogeneous integration of bulk TiNi material with silicon structures
and methods for cold-state reset and concepts for thermal energy supply.
The following sections will briefly introduce the challenges of integrating TiNi
material with silicon structures and the concept of knife gate microvalves. Finally,
first concepts to integrate bulk TiNi sheets and wires for actuating the valves will be
presented, together with initial measurements indicating an outstanding performance
of the TiNi actuated knife gate microvalves.
4.1
Integration of Titanium-Nickel shape memory alloy
Despite the advantages, the shape memory alloy Titanium-Nickel (TiNi) is not yet a
standard MEMS actuator material, partially due to the difficulties to integrate TiNi
with silicon structures. So far, TiNi is integrated either monolithically or in a hybrid
fashion.
In monolithic integration, the TiNi is deposited directly onto the silicon structures
using metal deposition techniques such as evaporation or sputtering. After deposition,
the film is amorphous, i.e. it displays no shape memory effect (SME) and is compressively stressed. To functionalize the layer, the film is crystallized in an annealing
step, which involves heating the film to typically above 535 °C [105]. The annealing
can even be performed by heating the substrate during deposition [106, 13]. During
crystallization the stress in the film relaxes and after subsequent cooling the film is
under tensile stress, which can be utilized to form a bimorph actuator. In the cold
state, the silicon structure (typically a cantilever) is stiff enough to overcome the tensile stress and remains flat, thereby deforming the TiNi and providing the cold-state
28
Wafer-level heterogeneous integration of MEMS actuators
(a)
(b)
Figure 14.
The thin SMA film actuated microgripper, consisting of two TiNi/silicon
bimorphs which are eutectically bonded together: (a) a schematic illustration [107] and
(b) a photograph of the device [21].
reset. Upon heating, the TiNi film contracts and forces the silicon cantilever to bend.
Upon cooling the silicon cantilever flattens the bimorph again. A well known example
for this technique is the microgripper [21, 107], which is shown in figure 14. In the
bimorph approach, the cold-state reset is provided by the built-in stress between the
TiNi and the silicon, thus, the cold-state reset is integrated monolithically with the
TiNi.
Another approach is to remove the silicon underneath the TiNi, which results in
a free standing TiNi film. This method requires additional elements to provide the
cold-state reset. Commonly, such additional elements include a bias spring and a
mechanical spacer, with the spacer placed in between the bias spring and the TiNi
film. During the assembly of the parts, the bias spring deflects the TiNi via the spacer.
When heated, the TiNi flattens and deflects the bias spring via the spacer. These
elements are integrated either on wafer-level by bonding of other wafers containing
the cold-state reset elements or on device-level in a hybrid fashion. Figure 15 shows
examples for both cases [108, 1].
Monolithic integration comes with several advantages, such as the wafer-level integration of the material and the possibility of a built-in cold-state reset when using
the bimorph approach. However, monolithic integration of TiNi also comes with some
major challenges [21]. The transformation temperature of TiNi is very sensitive to
variations of Ti or Ni content. An increase in the atomic percentage of nickel by
1% from an equi-atomic ratio of Ti:Ni lowers the martensitic start temperature from
50 °C to -100 °C. Therefore the deposition technique must provide precise and reliable
control of the deposition rate of the two different metals, which is difficult to achieve.
Even if a film has a proper global composition, local inhomogeneities result in the nucleation and growth of precipitates that inhibit shape memory behavior. The purity
of the TiNi targets and high deposition vacuum are essential to limit impurities in the
deposited film, which also potentially inhibit the SME. The high temperature annealing step causes compatibility problems with other materials and requires additional
layers to avoid unwanted interdiffusion between TiNi and silicon. Also, the large
4
KNIFE GATE MICROVALVES WITH BULK SMA MICROACTUATORS
(a)
29
(b)
Figure 15. Schematic drawings of TiNi actuated microvalves based on the free-standing
TiNi film approach with (a) silicon bias spring and spacer integrated via wafer-bonding [108]
and (b) external, pick-and-place integrated metal bias spring and saphire ball spacer [1].
thermal stresses after deposition and during annealing could cause delamination or
cracking of the films. Furthermore, the thicknesses of the films are limited to approximately 20 μm [5] and a recent report states that TiNi-based film sputtering is mostly
feasible for thicknesses up to 10 μm only [109], potentially resulting in limited mechanical robustness of structures actuated by TiNi films. For completeness it should
be mentioned that in a recent report [110] a 30 μm thick film of a ternary TiNi based
alloy was achieved by flash-evaporating TiNiCu onto copper substrates, however, the
issues of complicated processing and high post-deposition annealing temperatures are
still not overcome.
One method to address these issues is the hybrid integration, where the TiNi
is fabricated separately in dedicated factories and pick-and-place integrated as bulk
material on device level. Bulk TiNi is commercially available in different shapes
and dimensions, for example wires with diameters starting from 18 μm and thin
sheets starting from 20 μm (even thicknesses down to 5 μm have been reported [5]).
Similar to the free-standing thin TiNi film approach, the cold-state reset is provided
by additional elements, which are pick-and-place integrated on device level. As an
example, figure 16 shows a schematic drawing [111, 5] illustrating the principle of
hybrid TiNi integration for microvalves.
Hybrid integration of bulk TiNi comes with several advantages. The material is
produced with standard metalworking techniques in dedicated factories, resulting in
specified and reproducible material properties and thereby addressing the sensitivity
to variations of Ti and Ni content in the alloy. However, the pick-and-place integration of the TiNi in combination with the pick-and-place integration of the cold-state
reset elements is difficult for microdevices and potentially results in large fabrication
costs for larger volume production.
30
Wafer-level heterogeneous integration of MEMS actuators
Figure 16. Schematic illustration of hybrid integration of TiNi to fabricate a microvalve [111, 5]. The bulk TiNi element is patterned and pick-and-place integrated with
the other parts, including the spacer for the cold-state reset. An applied pressure pushes
the membrane up, which via the spacer deforms the TiNi element. When heating, the TiNi
pushes the membrane down to the flow orifice, thereby closing the valve.
Heterogeneous integration methods allow for the combination of the two methods
by integrating bulk TiNi material and providing a cold-state reset on wafer-level.
There is one report on heterogeneous TiNi integration [67], in which a SMA sheet was
patterned on wafer-scale and the elements were selectively transferred to single plastic
microvalves using the selective transfer integration approach of IBM [66] mentioned
above. However, the cold-state reset was provided by a spacer between the microvalve
and the SMA element, which again requires pick and place assembly. Large parts of
this thesis deal with wafer-level integration of both TiNi and the cold-state reset.
4.2
4.2.1
Gas microvalves
Background
A valve is a device that regulates the flow of gas or liquids. In this thesis, the focus
lies on millimeter sized valves for gas flow regulation. It would go beyond the scope of
this thesis to review all the research efforts into microvalves, therefore the interested
reader is referred to review papers [112, 113] for a more detailed overview.
Most of the active microvalves use mechanical moving parts which are coupled to
integrated actuators. Depending on the flow control principle, these microvalves can
be divided into diaphragm/seat type valves and gate type valves. Diaphragm/seat
type valves comprise a boss (most common is a flexible diaphragm) which moves in
parallel to the gas flow (see figure 17a) and which is coupled to an actuator. In the
closed state, the boss completely closes the orifice and the actuator must overcome
the pneumatic pressure, thus, . a strong actuator is necessary for controlling high
pressure. To allow for a large flow in the open state, the distance and thereby the
flow channel between the seat and the boss should be maximized, thus, the actuator
must provide a large stroke [114].
KNIFE GATE MICROVALVES WITH BULK SMA MICROACTUATORS
Boss
31
Guide
Guide
Guide
4
Gate
Guide
orifice
orifice
gas flow
gas flow
(a)
(b)
Figure 17. Illustration of: (a) seat valve with the flow regulating boss moving in parallel
to the gas flow, requiring a strong and large-stroke actuator; (b) gate valve with the flow
regulating gate moving perpendicular to the flow, reducing the requirements on the actuator.
In contrast to seat valves, the gas flow in gate valves is regulated by a gate which
moves perpendicular to the gas flow (see figure 17b). In the ideal case, when fully
opening the valve the gate is completely retracted out of the flow channel and there
is no obstruction in the flow path, resulting in higher flow rates compared to a similar
sized diaphragm/seat type valve. When moving the gate to close the flow path, the
pneumatic pressure is counteracted by the mechanical guidance of the gate and in the
ideal case the actuator is only moving the gate and must not counteract the pneumatic
pressure. However, the limited actuation energy available in microsystems does not
allow friction between sliding structures and therefore requires spacing between flow
orifice and gate, which results in leak flow in the closed valve state. Fortunately, many
valve applications tolerate leak flow.
Previously published work in the field of gate microvalves always featured gates
that move in the wafer plane [115, 116, 117, 118, 119]. Figure 18 [1] shows schematic
drawings of a sliding plate gate valve [115], which is currently commercialized by the
company Microstaq [120, 121, 122]. Yet, to allow the gate to move in the wafer-plane
requires extra footprint area and increases the footprint-related costs.
The silicon footprint consumed by the valve is a good cost indicator and when
comparing microvalves to each other not only the performance in terms of controlled
flow and pressure should be considered, but also the performance obtained per consumed footprint area. Therefore, in this thesis the comparison figure ’controlled
flow/pressure drop/footprint’ is suggested with the unit [sccm/kPa/mm2]. Table 4
shows the performance per footprint of different microvalves and the numbers indicate,
that gate microvalves typically have a much larger performance than diaphragm/seat
microvalves.
Microvalves should be fabricated cost-efficiently to allow them to be successful
outside the specific niche markets they currently focus on. This thesis presents the
knife gate microvalve concept, in which the gate moves out of the wafer plane to
minimize the chip footprint area and the footprint-related cost.
32
Wafer-level heterogeneous integration of MEMS actuators
Figure 18. Schematic drawings of the sliding gate microvalve introduced by Williams et
al [115], showing (a) a cross-section and (b) a three-dimensional drawing of the three silicon
wafers, which are assembled to form a pressure-balanced microvalve. The figure is copied
from [1].
4
KNIFE GATE MICROVALVES WITH BULK SMA MICROACTUATORS
33
Table 4. Comparison of different valves in terms pneumatic performance relative to their
consumed footprint area. It should be noted, that not all of the listed valves are silicon
valves, however, they are all microvalves. The performance comparison is related to the
knife gate valve presented in this work.
related to
Reference and actuation method
Q
P
w
l
Q/P/A
3400
95
2.3
2.3
4.21
valve in the
attached
paper 1
this work, knife gate microvalves
thermal bimorph, attached paper 1,
externally actuated
100%
SMA (TiNi sheet) actuation
1000
8
6.5
6.5
2.95
70%
SMA (TiNi wire) actuation
3800
50
2.0
4.5
8.44
200%
other gate microvalves
[115], thermal
6700
1000
2.5
5.0
0.54
13%
5000
100
4.0
4.0
3.13
74%
500
40
1.4
1.4.0
6.17
147%
5000
140
6.0
6.0
0.99
23.59%
1000
690
8.0
5.0
0.04
0.86%
1150
800
4.0
4.9
0.09
2.14%
100
170
2.5
2.5
0.09
2.24%
2000
200
10.0
10.0
0.10
2.38%
Yang et al [127], electrostatic
45
900
1.64
1.64
0.02
0.44%
Li et al [128], PZT
39
210
20.0
20.0
0.00
0.01%
Yang et al [129], PZT
52
2070
8.4
5.0
0.00
0.01%
Shao et al [130], PZT
70
50
13.0
13.0
0.01
0.20%
800
550
3.6
3.6
0.11
2.67%
Barth et al [132], thermal bimorph
1000
344
8.8
8.8
0.04
0.89%
Yanget al [133], thermopneumatic
1000
228
8.0
8.0
0.07
1.63%
3
16
3.0
8.0
0.01
0.19%
Kohl et al [135, 5], SMA (TiNi)
1600
120
20.0
11.0
0.06
1.44%
Kohl et al [136, 5], SMA (TiNi)
470
500
6.0
6.0
0.03
0.74%
Kohl et al [137, 5], SMA (TiNi)
180
210
6.0
11.0
0.01
0.31%
Kohl et al [137, 5], SMA (TiNiPd)
360
250
6.0
11.0
0.02
0.52%
Kohl et al [138, 5], SMA (TiNiCu)
360
300
6.0
11.0
0.02
0.43%
Kohl [5], SMA (TiNi)
280
200
6.0
6.0
0.04
0.92%
Kohl et al [139, 5], SMA (TiNi)
460
800
6.0
6.0
0.02
0.38%
[116], thermal
[119], externally magnetic
seat microvalves
Zdeblick et al [123], phase change,
(’FLUISTOR’ valve)
TiNi Alloy company, SMA (TiNi),
figure 15, [1]
Messner et al [124], thermal bimorph
Jerman [125], thermal bimorph
Fu et al [126], magnetic
Huff et al [131], electrostatic
Bosch et al [134], magnetic +
electrostatic
Q = flow in sccm; P = pressure in kPa; w,l = width, length in mm; A = w×l = footprint in mm2
Q/P/A = performance per footprint in sccm/kPa/mm2
34
Wafer-level heterogeneous integration of MEMS actuators
ga
actuator
gas
channel
gas
channel
gas
channel
actuator
actuator
s
gate
gas
(a) front gate valve
gate
ga
s
gate
(b) side gate valve
(c) back gate valve
Figure 19. Illustration of the three different knife gate microvalve designs introduced in
detail in the attached paper 1.
4.2.2
Knife gate microvalves
This thesis presents the knife gate microvalve concept, in which the gate moves out
of the wafer plane to minimize the chip footprint area and the footprint-related costs.
Figure 19 illustrates the three different designs of microvalves with the gate moving out-of-plane. The gate is coupled to a monolithically integrated thermal silicon/aluminum bimorph actuator and moves out-of-plane to regulate an in-plane flow.
The valve concept is explained in detail in the attached paper 1.
Figure 20 confirms the valve’s potential for controlling large flows: an active chip
area of only 2.3×3.7 mm2 allows for a flow change of ΔQ = 3400 standard cubic
centimeters per minute (sccm) at a pressure difference of 95 kPa.
Table 4 lists the performance per footprint of different microvalves. For simpler
comparison the presented knife gate microvalve is chosen as reference and all the
other microvalves are compared to it. The comparison shows that the knife gate
microvalve provides the highest performance per footprint, only outperformed by a
recently published rotary gate valve and the TiNi wire actuated knife gate microvalve.
The rotary valve is electromagnetically actuated using an external magnetic field and
belongs therefore to another class of microvalves with external actuation mechanisms.
5000
100% open
flow [sccm]
4000
50% open
3000
2000
closed
1000
0
20
40
60
pressure [kPa]
80
leak flow from imperfect assembly
leak flow from gate
spacing
Figure 20. Measured pressure-flow characteristics of a back-gate microvalve. The dashed
line indicates the expected leak caused by imperfect assembly.
4
KNIFE GATE MICROVALVES WITH BULK SMA MICROACTUATORS
35
polymer bonding
layer
TiNi sheet actuator
gate
base
gas flow
carrier substrate
eutectic bonding layer
(a)
locally remove bonding layer
(b)
(c)
Figure 21. Cross-sectional drawing of the first design of a TiNi sheet actuated knife gate
valve showing (a) a back gate valve bonded to a carrier substrate and the TiNi sheet prior
to integration; (b) the adhesive bonding of the TiNi sheet onto the valve and (c) releasing
the flow regulating gate to allow for its manipulation by the actuator.
The TiNi wire actuated knife gate microvalve is a successor of the thermal bimorph
knife gate microvalve and will be discussed below.
However, it should be noted that while the experiments indicate the excellent
capabilities of the knife gate microvalve concept, the thermal bimorph actuator failed
for several reasons. First, the bimorph beam was not strong enough to withstand
the vibration and torsional forces caused by the flow. Second, improper thermal
design caused improper heating and actuation of the beam. Bulk TiNi actuators were
identified as a promising replacement, however, concepts for integrating bulk TiNi
materials had to be developed first.
4.3
4.3.1
TiNi sheet actuated knife gate valves
Design
In a first, yet unpublished design to integrate TiNi sheets onto silicon knife gate
microvalves, the TiNi is integrated with a back gate valve (back gate valve see figure 19c). Figure 21 shows the design, in which the back-gate valve is bonded onto
a carrier substrate with a vertically through-etched hole, providing an enclosed flow
channel which is expected to simplify the pneumatic connection. The TiNi is bonded
onto the valve: one end of the TiNi is bonded to the flow channel forming the base
for the cantilever and the other end is bonded onto the flow-regulating gate.
4.3.2
Fabrication - Integration of TiNi sheets
A method was investigated to adhesively bond TiNi sheets onto silicon structures, together with methods for the cold-state reset and thermal energy supply. The attached
paper 2 contains a detailed description of these methods.
The concept to integrate TiNi sheets onto silicon structures is illustrated in figure 22a. The TiNi sheet is patterned to form the actuator structures, using flexure
interconnections to hold them together. The patterning is performed prior to integration to avoid the exposure of the silicon target structures to the very aggressive TiNi
etchant. The bonding adhesive is stamped onto the surface of the patterned target
36
Wafer-level heterogeneous integration of MEMS actuators
wafer- sized, bulk
TiNi sheet
patterning of
TiNi sheet
flexures holding
cantilevers together
aligning to target wafer
target wafer coated
with adhesive layer
bonding to target wafer
etched holes to allow
for cantilever movement
(a) TiNi sheet integration concept
cold state
stress layer
hot state
resistive heater
bulk TiNi
(b) trimorph actuator concept
Figure 22. Illustration of (a) final bulk TiNi sheet integration concept and (b) trimorph
actuator concept, as developed in paper Nr. 2.
wafer, which was a ’dummy’ wafer with etched holes to allow the TiNi actuators to
deflect freely. The stamping method is specially developed in this thesis and based
on standard dicing blue tape. Then, the TiNi sheet is aligned and bonded onto the
target wafer. For developing this integration method, the target wafer was a ’dummy’
wafer with etched holes, which allow the TiNi actuators to deflect freely.
After the TiNi material is integrated, the cold-state reset and thermal energy
supply must be provided for actuation. In this work, these elements are integrated
monolithically with the TiNi sheet in form of three functional layers. Figure 22b
illustrates the actuation concept. The first functional layer is the core actuation layer
formed by bulk TiNi material. The second functional layer is a stressed film which
deforms the actuator in the cold-state. This film is deposited onto the TiNi and
eliminates the need for pick-and-place integration of an additional bias spring. The
third functional layer is a heating layer to supply thermal energy to the actuator via
an indirect heating scheme and to avoid the complicated electrical contacting of the
TiNi alloy.
In the valve design shown in figure 21, the TiNi sheet is bonded onto the gate,
which itself is bonded to another silicon wafer. To allow the gate to be manipulated for
controlling the flow, its underlying bonding layer must be removed without removing
the bonding layers underneath the other silicon structures. Section 3.2.4 presents some
methods for the localized release of structures while keeping other structures bonded,
4
KNIFE GATE MICROVALVES WITH BULK SMA MICROACTUATORS
flow [sccm]
1800
37
open state
1400
1000
600
closed state
200
0
5
10
pressure [kPa]
15
Figure 23. First measurements of the pressure-flow characteristics of a yet unpublished
TiNi-sheet actuated knife gate microvalve.
however, most of them require underetching of the structures to be released, including
exposure of the whole device to harsh environments and etching of all bonding layers.
To avoid these issues, a new bond-and-release method was developed: The valve
and the carrier substrate are bonded together using eutectic Au-Si bonding, followed
by localized removal of the eutectic bonding layer. For a detailed description and
investigation of this method, the reader is referred to the attached paper 4.
4.3.3
Results
Figure 23 shows first results of the flow performance of a gate microvalve with TiNi
sheet actuation. An active chip area of only 6.5×6.5 mm2 allows for a flow change of
ΔQ ≈ 1000 sccm at a pressure difference of 8 kPa. These numbers result in a flow
performance per footprint (see Table 4) of about 70% of the first knife gate valve with
the non-functional thermal bimorph actuators. However, in contrast to the first knife
gate valve, the TiNi sheet actuated valve was actuated with the integrated actuator
and did not require any external actuation. Furthermore, the tested TiNi sheet knife
gate valve is an early prototype with room for optimization.
4.3.4
Discussion
The TiNi sheets are integrated directly onto the silicon structures, without any intermediate carrier or transfer substrate. Since the sheet is patterned prior to integration,
it has to be aligned to the target wafer. Both alignment and placing onto the adhesive
layer were performed manually. However, the patterned TiNi sheet was fragile and
very easily deformable, which complicated the handling. The alignment of the sheet
was enabled only by a high ambient temperature caused by a hotplate underneath
the target wafer, which triggered the stiff hot state of the sheet. To allow for an
alignment and bonding using a standard wafer alignment and bonding tool, future
designs should aim for a transfer integration method, in which the SMA sheet is preprocessed on an intermediate carrier substrate and then transfer bonded from the
carrier substrate onto the target wafer.
38
Wafer-level heterogeneous integration of MEMS actuators
probe needles
TiNi cantilever
silicon
eutectic Au-Si
bonding area
heater
TiNi cantilever
with stress layer
(a)
(b)
Figure 24. (a) Photographs of a eutectically bonded TiNi cantilever which was severely
plastically deformed by manual pulling with a tweezer without breaking the bond, thereby
illustrating the extreme bond strength. (b) Picture of a eutectically bonded TiNi trimorph
actuator with a resistive heater.
The sheets are affixed onto the silicon structures using adhesive polymer bonding.
The adhesion strength looks promising, however, more tests are necessary to verify the
bond stability, especially for long-term cycling of the actuator. An alternative method
could be the one used for the micromirror arrays presented in section 3: the adhesive
bonding could serve as temporary mechanical fixation and electroplated metal vias
through the TiNi sheets could provide both electrical interconnection and permanent
mechanical fixation. Yet another alternative is to use a different bonding method
which allows for direct mechanical fixation. Figure 24a shows non-published pictures
of first experiments testing the eutectic bonding of TiNi to silicon and illustrating the
extreme bond strength: a eutectically bonded TiNi cantilever was severely plastically
deformed by manual pulling with a tweezer without breaking the bond.
At this stage, an electrical interconnection between the TiNi and the silicon structures is not necessary, since any scheme for electrical contacting of the actuator would
most likely involve wire bonding directly to the metal heater on top of the TiNi trimorph actuators. Since the work in the attached paper 2 was focused on integration
and cold-state reset, the test structures did not contain a metal heater. Figure 24b
shows a photograph of a yet unpublished complete trimorph actuator containing a
resistive metal heater, which is eutectically bonded to silicon. The heater was contacted using probe needles and consumed approximately 100 mW to trigger the TiNi
hot state.
4.4
4.4.1
TiNi wire actuated knife gate valves
Concept
Figure 25 shows a first, yet unpublished design to integrate TiNi wires with the knife
gate valve concept. The wires are integrated with a front gate valve (front gate valve
see figure 19c).
4
KNIFE GATE MICROVALVES WITH BULK SMA MICROACTUATORS
39
polymer bonding
TiNi wire
gate
gas flow
(a)
(b)
(c)
Figure 25. Cross-sectional drawing of the first design of a TiNi wire actuated knife gate
valve showing (a) a front gate valve and the TiNi wires prior to integration; (b) the adhesive
bonding of the TiNi wires onto the valve and (c) the actuation.
4.4.2
Fabrication - Integration of TiNi wires
For the SMA wire actuators, the integration concept is derived from the actuation
concept, which is illustrated in figure 26a. TiNi wires are strained by pulling them
prior to integration. Then, the strained wires are integrated with silicon cantilevers
by anchoring one end of the wires to the base of the silicon cantilever and the other
end of the wire to the moveable end of the cantilever. Upon heating, the TiNi wires
recover the strain, i.e. they contract and thereby force the silicon cantilever to bend
out of plane. In the cold state, the silicon cantilevers act as cold-state reset, recovering
their flat shape by stretching the TiNi wires.
Figure 26b illustrates the resulting wafer-level integration concept. The wires are
oriented and placed on a dedicated tensioning frame, which is used to strain the
wires. Then, the wafer and the wires are aligned to each other and put in contact.
Finally, the wires are anchored for each actuator and the tensioning frame is removed.
After integration, the wires are diced together with the silicon chips. For a detailed
investigation including the proof of concept by fabricating first test structures the
reader is referred to the attached paper 3. The fabricated test devices showed strokes
of up to 396 μm for a cantilever length of 3 mm, which is among the highest values in
comparison to different microactuators reported in a recent review paper [6]. Since
the work in this paper was focused on integration and cold-state reset, no heating
concept is presented.
4.4.3
Results
Figure 27 shows first results of the flow performance of a knife gate microvalve with
TiNi wire actuation. An active chip area of only 2×4.5 mm2 allows for a flow change
of ΔQ ≈ 3800 sccm at a pressure change of 50 kPa. These numbers result in a
flow performance per footprint (see Table 4) of about 200% of the first knife gate
valve with non-functional thermal bimorph actuators. In contrast to the first knife
gate valve, the TiNi wire valve is actuated with the integrated actuator. The flow
performance per consumed footprint area is a world record compared to the other
valves and clearly demonstrates the promise of TiNi actuated knife gate microvalves.
4.4.4
Discussion
The TiNi wires are transfer-integrated, using a dedicated tensioning frame as temporary carrier. For the first test devices, the alignment was performed manually and the
wires were placed onto the adhesive anchors using a dedicated lift stage. This process
40
Wafer-level heterogeneous integration of MEMS actuators
operational states of the actuator
TiNi wire
anchor
strained TiNi wire
anchor
silicon
straining wire
cold state
hot state
(a) actuator concept
fixed clamp
polymer anchors
moveable clamp
TiNi
wires
target wafer with
silicon cantilevers
clamping wires
straining wires
aligning and contacting
anchoring
(b) wafer-level integration concept
Figure 26. Illustration of (a) the actuation concept using TiNi wires on silicon cantilevers
and (b) concept for wafer-level integration of the wires, as developed in the attached paper 3.
open state
flow [sccm]
6000
4000
closed state
leakage due to
imperfect assembly
2000
0
0
10
20 30 40
pressure [kPa]
50
theoretical
flow lines
Figure 27. First measurements of the pressure-flow characteristics of a yet unpublished
TiNi-wire actuated knife gate microvalve.
4
KNIFE GATE MICROVALVES WITH BULK SMA MICROACTUATORS
electrical connector pins
41
TO 8 housing
wirebonding
valve
pneumatic connector tube
Figure 28. Illustration of the packaging concept for the knife gate microvalves. The valve is
packaged in a standard TO8-housing, like it is used for pressure sensors. The Picture shows
a pressure sensor of the Robert Bosch GmbH with the lid removed.
could be adapted to implement a high precision (x,y,z) stage, thus, the process looks
promising for the wafer-level integration of wires. However, prior to integration the
wires need to be placed onto the tensioning frame with the correct orientation and
pitch, which was performed manually in this work. Future work should verify that
the wires can be placed onto the tensioning frame using automated methods.
The wires are attached to the silicon cantilevers using anchors made of SU-8, which
is locally cured with UV light. However, the area underneath the wires is shadowed
by the wires and therefore not fully cured. Future work should implement reflective
patterns in the silicon underneath the wires to reflect the UV light into the SU-8
volume underneath the wires. Alternatively, the wires could be anchored using metal
clamping via electroplating metallic connections between the silicon and the wires.
4.5
Outlook
The work presented above is a big step towards a microvalve with potential for commercial application. However, a fully integrated TiNi actuated valve with integrated
heaters has not been shown so far. All necessary elements have been discussed and
future work will focus on combining all elements to show a fully functional TiNi
actuated microvalve.
For commercialization the valve must be packaged. Figure 28 shows the suggested
packaging concept, which is inspired by the packaging of several commercial pressure
sensors. The valve is glued onto the pneumatic connector tube of a TO8-housing,
thereby pneumatically interconnecting the valve to an inlet pressure. The outlet of
the flow can be provided by a hole in the lid of the TO8-housing. The electrical interconnection to the actuator on the valve is provided by wirebonding to the electrical
interconnection pins of the TO8-housing.
42
Wafer-level heterogeneous integration of MEMS actuators
5 AUTOMATED MAIN DISTRIBUTING FRAMES WITH S-SHAPED
ACTUATOR SWITCHES
5
43
Automated main distributing frames with S-shaped
actuator switches
This chapter presents a MEMS crossbar switch as the core unit to automate main
distributing frames, in which the parties in copper-wire telecommunication networks
are cross-connected. The crossbar switch consists of an array of MEMS switches
which are individually addressable and allow to interconnect a number of input lines
to a number of output lines in any combination desired. In this work, the switches
are based on the electrostatic S-shaped actuator described earlier in section 2.3.2.
Each of the switches in the array is individually addressable using a row/column
addressing method, avoiding a complicated CMOS circuitry as utilized in micromirror
arrays. The fabrication concept of the crossbar switch is based on the basic concept
of heterogeneous integration, which involves the separate fabrication of parts followed
by their assembly to a device.
The following subsections briefly introduce automated main distributing frames,
followed by a description of the S-shaped actuator switch. Finally, the MEMS crossbar
switch and the concept of individual switch addressing are presented.
5.1
Switch units in automated main distributing frames
The current telecommunication networks evolved from the original telephone networks and they are for the most part still based on copper-wire networks. Within
these networks, the main distributing frame (MDF) is the center of cross-connecting
equipment of service providers to their customers and it is located both in large central
offices and remote street cabinets. The MDF still relies upon the same routines and
equipment it did 50 years ago. Hence, whenever a cross-connection must be changed,
this re-configuration must be carried out manually by technicians physically changing the interconnection wires (jumpering) in the MDF. Before the deregulation of the
telecommunication markets the customers only rarely changed their services and there
were no or little economical incentives to automate the jumpering. Nowadays, however, the customers are highly flexible and there is a growing need to adapt quickly to
new telecommunication services. Due to the increased labor-intensive manual jumpering the network maintaining costs are drastically rising and thereby creating a clear
incentive to automate the jumpering using an automated MDF (AMDF).
The core element of the AMDF is a crossbar switch. Originally, the term ”crossbar
44
Wafer-level heterogeneous integration of MEMS actuators
+
+
+
+++output channels
(a) electrical diagram of
a switch unit
input channels
double
switch
output channels
input channels
crossbar
switch unit
(b) multicast switching networks
Figure 29. Electrical and hierarchical schematic diagrams: (a) switch unit, consisting of
an array of 3 × 3 double switches; (b) arrangement of switch units on higher hierarchical
layers using the Clos method to minimize the necessary amount of switches.
switch” relates to a grid of metal bars crossing each other without touching each other
and which are electrically interconnected or isolated by switches in their crosspoints.
In a AMDF, the metal bars are formed by signal channels, with a set of input channels
and a set of output channels. A channel consists of a signal line pair and switching
the signal line pair simultaneously requires a double-pole single-throw (DPST) switch
in each cross-point. Typically, the cross-bar switches are symmetrical, with N inputs
and N outputs resulting in N2 crosspoints. Figure 29a shows an electrical schematic
of the signal circuit for a simplified crossbar switch with N=3. A typical AMDF
connects tens of thousands of channels and it is not feasible to provide this amount of
crosspoints in a single crossbar switch since it is difficult to fabricate such large arrays
with an acceptable yield. Furthermore, each bar or channel can only be connected in
one crosspoint while the other crosspoints remain unused and therefore large parts of
the crossbar switch are not utilized. A solution is to fabricate small crossbar switch
units with a low N and interconnect several of them to a bigger crossbar switch
unit, which itself again forms a switch unit in a bigger crossbar switch. In 1953,
Charles Clos at Bell labs developed the ’minimal spanning switch’ method [140] to
interconnect the small crossbar switch units in a way to reduce the total number of
switches and interconnections, but still allowing for a non-blocking network which
always provides one route to interconnect an input to an output. In this method, the
switch units are arranged in three stages with each the same number of switch units
and each switch unit sharing only one connection with a switch unit in another stage,
as illustrated in figure 29b. If a path is blocked, the connections can be rerouted
via other switch units to open a new path. When scaling up the resulting crossbar
switch, Clos’ interconnection method reduces the√ amount
√ of
√ crosspoints from the
quadratically scaling N2 to the more favorable 3× N × N × N = 3×N1.5 , thereby
drastically minimizing the number of switches and the related costs. As an example,
a conventional crossbar switch with N = 100 would require N2 = 10000 crosspoint
switches, whereas a Clos crossbar switch requires 10 switch units à 10×10 crosspoints
5 AUTOMATED MAIN DISTRIBUTING FRAMES WITH S-SHAPED
ACTUATOR SWITCHES
45
Figure 30. Illustration of the switch unit in a robot based AMDF: The signal channels
are interconnected in their crosspoints by inserting/extracting metal pins using a robot on
a xy-stage. The figure is copied from [145].
in three stages, which results in 10×10×10×3 = 3000 crosspoint switches, only.
It would go beyond the scope of this thesis to extend this introduction, therefore
the interested reader is referred to literature such as [140, 141, 142, 143, 144].
A crossbar switch unit comprises three basic elements. The first element is the
routing network with input and output signal channels crossing each other without
touching each other. The second element are switches needed for interconnecting the
signal channels in their crosspoints. The third element is controlling of the switches:
each switch must be individually addressable to allow for the interconnection of any
of the N input lines to any of the N output lines in any configuration desired.
To ensure the highest flexibility when connecting new services and new technological equipment to the customers, the signal path should ideally act like a copper-wire
cable which requires ohmic mechanical switching with metal contacts and excludes
solid-state relay based crossbar switching. Previous attempts to provide a switch
unit for a Clos network in an automated MDF consist of robots [145, 146, 147] or
electromagnetic relays [148, 149].
Figure 30 [145] exemplarily illustrates the robot approach: the routing network is
formed by a matrix board and the switches are metal pins, which are inserted/extracted
in the crosspoints of the signal channels using a robot. The addressing is provided by
moving the robot to the addressed crosspoint using a xy-stage with a laser for fine
alignment. In the electromagnetic relay approach, the crossbar switching is performed
by an array of electromagnetic relays on printed circuit boards. These arrays include
control circuits with diodes and transistors to allow for a row/column addressing.
The attached paper 5 contains a short comparison of the technologies, which did not
provide solutions with the desired performance at acceptable costs
Nowadays, MEMS technology offers the possibility to integrate a large number of
micro-switches on a single chip and utilizes high-volume semiconductor manufactur-
46
Wafer-level heterogeneous integration of MEMS actuators
top electrode stiff cantilever
bottom
electrode
switching
contact
large gap between
electrodes
(a) OFF
(b) ON
Figure 31. Illustration of a conventional cantilever-type electrostatic MEMS switch. The
figure is modified from [37].
ing methods and logistics which potentially results in low production costs. MEMS
switches have been investigated extensively during the last decade for many applications requiring high quality signal switching [150, 151, 152]. They fill the gap between
solid-state relays (SSR) and electromagnetic relays (EMR) by offering true ohmic
switching, high miniaturization and integration and very good signal properties over
a large bandwidth. To the authors’ knowledge, there are no scientific reports on
MEMS crossbar switches or switch arrays for the described application. However,
there are a few reports on small-scaled and unpackaged metal-contact MEMS switch
arrays for other applications [153, 154, 155, 156, 157, 158].
5.2
5.2.1
MEMS switches
Background
MEMS switches are devices that mechanically open or short-circuit a transmission
line. They are of submillimeter size and they outperform semiconductor switches in
terms of power consumption, on-state resistance, cut-off frequency, isolation and signal
insertion loss [150,151,152]. There are two basic types of MEMS switches: capacitive
switches and metal-contact switches. Capacitive switches are typically used as shunt
switches between the signal line and the ground line while metal-contact switches are
used as series switches between two transmission lines, opening or closing the signal
path. In metal-contact switches, the signal is conducted through metal contacts and
in capacitive switches, the contact is made between the metal and dielectric material.
The application in automated MDF’s described above requires ohmic switching of
signals from DC to max. 50 MHz, therefore only metal contact series switches are of
interest in this work.
For metal contact switches there are several requirements to consider when designing the switch. In the off-state, the switch should provide a high isolation when
applying signals with high voltages, hence, a large distance between the switch contacts is necessary. When the switch is closed, the contact resistance should be as
low as possible and the contact should remain stable during the whole ON time. The
switch contact resistance and reliability is directly influenced by the contact force and
the contact material. The influence of the contact materials is a research field on its
own [159, 160, 161, 162] and not touched upon in this work. The contact force should
be maximized, however, a strong contact force increases the risk of stiction between
5 AUTOMATED MAIN DISTRIBUTING FRAMES WITH S-SHAPED
ACTUATOR SWITCHES
47
Figure 32. Illustration of the connection between the design parameters and the active
forces in the switch model and how they affect its performance in terms of contact resistance
and reliability. The figure is copied from [37].
the contacts. Therefore, the forces opening the switch should be strong enough to
overcome the adhesion forces between the switch contacts.
Most of the MEMS switches are electrostatically actuated and the conventional
and so far most common electrostatically actuated MEMS switch is based on a simple
cantilever approach, as illustrated in figure 31. The switch contact bar at the moveable end of the cantilever interconnects two signal lines when the cantilever is pulled
down, as illustrated in figure 31b. This configuration is basically a parallel-plate
actuator and comes with all the problems mentioned in section 2.3.1, especially in
combination with the switch requirements described above: a large distance between
the switch contacts for a high OFF state signal isolation results in a large initial electrode gap and thereby in large actuation voltages. To overcome stiction forces when
opening the switch, a stiff cantilever is required to provide strong opening forces.
As a consequence, increased actuation voltages are necessary to overcome the strong
spring force. Figure 32 [37] illustrates all the parameters involved for electrostatically
actuated metal contact switches and how they are interconnected.
5.2.2
The S-shaped actuator switch
To overcome the limitations of the conventional cantilever type switch, an alternative approach was developed based on the S-shaped film actuator described in section 2.3.2 [35, 36, 37]. The assembly concept of the S-shaped actuator highlights this
actuator for the heterogeneous integration with other structures. One substrate contains all the moveable parts of the actuator, while the target substrate provides an
actuation electrode and a spacer to provide the gap in which the membrane moves
up and down. Figure 33 illustrates the basic concept of the S-shaped film actuator
48
Wafer-level heterogeneous integration of MEMS actuators
switch substrate
switch contact
signal lines to connect
d
V
target substrate
(a) before assembly
(b) after assembly: OFF
(c) ON
Figure 33. The S-shaped actuator switch: (a) before assembly, (b) after assembly in the
OFF-state and (c) in the ON-state.
switch. The switch substrate contains the flexible membrane with a switch contact
and the target substrate contains the signal lines which are to be interconnected by
the switch (figure 33a). The distance d between the switch contacts in the OFF state
(figure 33b) can be tuned and a large distance allows to obtain a high off-state isolation. In the ON state (figure 33c), the signal lines are interconnected by the switch
contact on the membrane. The switch contact area is surrounded by the actuation
electrode, resulting in a more homogeneous contact force.
If the spring force of the thin rolling membrane is not strong enough to open the
switch, the upper zipper-actuator provides an active opening capability.
5.3
The MEMS crossbar switch
The following subsections introduce the MEMS crossbar switches developed in this
thesis. These crossbar switches are fully fabricated using standard MEMS technologies and materials. Thus, strictly speaking no heterogeneous integration is applied
since heterogeneous integration refers to the integration of different technologies or
materials. However, the development of the MEMS crossbar switch is based on the
basic concept of heterogeneous integration; the device is divided into substructures,
which are fabricated separately and finally integrated with each other.
For the MEMS crossbar switch, the elements to integrate are a signal channel
routing network, switches to interconnect the signal channels in their crosspoints and
a control line network for the individual addressing of single switches. The following
sections briefly introduce the routing network, the switches and their integration to
a crossbar switch. Finally the addressing concept will be presented. A detailed
description of the concept and the fabrication can be found in the attached paper 5
and 6.
5.3.1
The routing network
Figure 34 (bottom part) illustrates the signal line routing network. The signal lines
are fabricated in two different metal layers, which are isolated from each other by an
intermediate polymer layer. The input lines are in the lower metal layer and the output lines are in the upper metal layer, crossing the input lines. In each crosspoint the
input lines are connected to the upper metal layer by vertical metal vias through the
5 AUTOMATED MAIN DISTRIBUTING FRAMES WITH S-SHAPED
ACTUATOR SWITCHES
top part with switches to
interconnect signal lines
in their crosspoints
double switch in
the crosspoints
assembly
49
membrane
electrode
top
electrode
spacer ring and
adhesive layer
assembly
intermediate
isolation layer
sign
s
elec ignal al trod +
e
input on lower
metal layer
al +
sign nal sig
vertical via through
intermediate isolation
layer
crosspoint of
input and output
output + bottom electrode
on upper metal layer
bottom part with signal
line routing network
Figure 34. Three-dimensional drawing showing the concept of the KTH MEMS crossbar
switch, simplified to a 2 × 2 array. Some components of the bottom part have been removed
to reveal details. The bottom part contains the signal-routing network on two metal layers
and the bottom electrodes for the switch actuation. Furthermore, the picture shows the two
BCB layers for isolating the metal layers of the routing network from each other and for
creating the encapsulating spacer ring between the top and the bottom part. The top part
contains the moving part of the S-shaped film actuator switches with the switch contacts to
interconnect the signal lines in their crosspoints.
50
Wafer-level heterogeneous integration of MEMS actuators
intermediate polymer layer. The upper metal layer also contains actuation electrodes
(bottom electrodes) to actuate the switches later on.
5.3.2
The crosspoint switches
Figure 34 (top part) illustrates the switching part of the crossbar switch, which allows
to interconnect the signal lines in each crosspoint. The switches are S-shaped actuator
switches as introduced above. As illustrated in the electrical schematic of the signal
circuit in figure 29a, the signal lines are paired to channels, consisting of a signal+
and a signal− line. To switch both lines simultaneously, each membrane contains two
separate switch contacts to allow for the parallel switching of two line pairs in the
target substrate.
5.3.3
The integration
Finally, the top part with the switches is placed onto the bottom part with the signal
channels. The spacing between the two parts is provided by a polymer ring (figure 34,
spacer ring) around the crosspoints. Besides spacing, the polymer ring is also designed
to provide the mechanical fixation of the two parts. The polymer ring is patterned
prior to the assembly, but not fully cured. After the assembly, the polymer is cured
and thereby the two parts are adhesively bonded. Furthermore, the switches are
encapsulated/packaged by the polymer ring, based on the encapsulation/packaging
concept illustrated in figure 10.
5.3.4
Individual switch addressing
There are several possibilities for the individual addressing of the switches. The
conventional approach would be to connect each switch with an individual control
wire. However, the number of switches and thereby the number of control wires scale
quadratically with the size of the matrix, making this conventional approach feasible
only for very small arrays with a small number of elements. Another approach is
to integrate a CMOS addressing circuitry as shown earlier for micromirror arrays,
which allows for the analog control of the actuators in the array as well as for high
addressing frequencies [50]. The presented fabrication concept of the MEMS crossbar
switch would allow for the heterogeneous integration of the switches with a CMOS
wafer, however, the AMDF application requires only digital mode addressing with
low addressing frequencies and does not justify the integration of high-cost CMOS
circuits.
The addressing approach developed for the MEMS crossbar switch utilizes the
hysteresis behavior of the actuation voltages of electrostatic actuators, which was
explained in section 2.3.1 and illustrated in figure 3c. Applying a set voltage Vset
which is larger than the pull-in voltage always pulls in the moveable plate and the
switch is ON: Vset ≥ Vpull−in . Reducing the applied voltage to a hold voltage Vhold
within the hysteresis range between the pull-in and the pull-out voltage holds the
moveable plate in the current position, i.e. either in the ON or in the OFF state:
5 AUTOMATED MAIN DISTRIBUTING FRAMES WITH S-SHAPED
ACTUATOR SWITCHES
51
-Vset-
Vhold Vhold Vhold
Vpull-out < Vhold < Vpull-in
Vhold Vhold Vhold
+
Vset+
Vhold + Vset+ - (-Vset-) > Vpull-in
Vhold - (-Vset-) < Vpull-in
Vhold Vhold Vhold
Vpull-out < Vhold < Vpull-in
Vhold + Vset+ < Vpull-in
Vpull-out < Vhold < Vpull-in
(a)
(b)
(c)
Figure 35. Schematic drawings illustrating the individual setting of cell (2,1) in a 3×3
array and the voltages involved. (a) Hold state. Each actuator maintains its current state.
(b) Pull-in of the actuator by applying the addressing voltages at the respective rows and
columns. (c) Return to hold state. Maintaining the programmed states of all cells.
Vpull−out < Vhold < Vpull−in . To open the switch, the applied voltage is reduced to
the reset voltage Vreset , which is smaller than the pull-out voltage: Vreset ≤ Vpull−out .
For the addressing scheme to work, the electrodes of the fixed plates are interconnected in each respective column, whereas the electrodes of the moveable plates
are interconnected in each respective row. This arrangement allows to provide the
required potential differences in the crosspoints of the electrodes and the number
of necessary control lines is reduced from the quadratically scaling N2 to the linear
scaling complexity of 2N. As an example, with the conventional addressing approach
a crossbar switch with N = 20 would require 400 control lines, while the presented
row/column approach requires only 2N = 40 control wires.
The individual pull-in is illustrated in figure 35. If all columns are fed with the
hold voltage Vhold and all rows are on ground potential, all switches maintain their
current states (figure 35a). To set the actuator at the address (2,1), the potential
difference between the electrodes must exceed the pull-in voltage Vpull−in . Therefore,
a voltage level of Vhold + Vset+ is applied at the 2nd row, and a negative voltage level
of Vset− is applied at the 3rd column (figure 35b). After the individual pull-in the
voltages ΔVset+ and ΔVset− are removed and the columns are connected to the hold
voltage Vhold , maintaining the current state of all switches as illustrated in figure 35c.
The individual pull-out is illustrated in figure 36. All columns are fed with the hold
voltage Vhold and all rows are on ground potential, all switches maintain their current
states (figure 36a). To reset the actuator at the address (2,1), the potential difference
between the electrodes must be below the pull-out voltage Vpull−out . Therefore, a
positive voltage level of Vreset is applied at the 3rd column and thereby subtracted
52
Wafer-level heterogeneous integration of MEMS actuators
Vreset
Vhold Vhold Vhold
Vpull-out < Vhold < Vpull-in
Vhold Vhold Vhold
+
+
Vreset
Vreset
Vhold - Vreset < Vpull-out
Vhold Vhold Vhold
Vpull-out < Vhold < Vpull-in
Vpull-out < Vhold < Vpull-in
Vhold + Vreset - Vreset = Vhold
Vhold +Vreset < Vpull-in
(a)
(b)
(c)
Figure 36. Schematic drawings illustrating the individual resetting of cell (2,1) in a 3×3
array and the voltages involved. (a) Hold state. Each actuator maintains its current state.
(b) Pull-out of the actuator by applying the addressing voltages at the respective rows and
columns. (c) Return to hold state. Maintaining the programmed states of all cells.
from Vhold . To avoid resetting all the switches in the 3rd column, Vreset must be
added to Vhold at the other rows (figure 36b). Finally, all columns are connected to
the hold voltage Vhold , maintaining the current state of all switches as illustrated in
figure 36c. Another simpler method for the resetting is to connect all electrodes to
ground and set a new pattern as described above.
This addressing scheme has been reported before for micromirror arrays [163,164,
165,166,167] and programmable capacitor banks [168]. Neither of these reports, however, discusses any design requirements or considerations on the operational robustness of the addressing scheme. This thesis contains a full investigation for determining
the operational reliability of the addressing scheme and to optimize the addressing
voltage levels, taking into account the statistical deviations of the actuation voltages
both for a single actuator as well as across the array. The attached journal paper 6
extensively investigates this issue and presents methods to choose actuation voltages
which result in a statistical certainty of better than 99.9% for the addressing to work.
5.4
Discussion
The routing network and the switches are integrated directly, without any intermediate carrier or transfer substrate. The mechanical fixation of the two substrates is
designed as adhesive bonding via the patterned adhesive spacer ring. In this design,
the polymer spacer ring not only provides spacing and mechanical fixation, but also
an encapsulation and thereby packaging of the crosspoints. However, it should be
noted here, that even though the mechanical fixation of the two substrates is designed as a wafer-level process, the first test structures were assembled manually on
5 AUTOMATED MAIN DISTRIBUTING FRAMES WITH S-SHAPED
ACTUATOR SWITCHES
53
Figure 37. Picture of the electrical contacting to evaluate the crossbar switch. The arrow
indicates the probe needle from the bottom side to contact the electrodes on the top substrate
of the crossbar switch.
chip-level using epoxy droplets in the corners for mechanical fixation. This issue will
be addressed in more detail below.
Usually, a controlled atmosphere inside the packaged volume is desired and therefore hermetic sealing of the encapsulation would be the optimum. The polymer based
sealing of the presented crossbar switches does not provide hermetic sealing even if
the chosen polymer, Bencocyclobutene (BCB) from the Dow Chemical Company, is
often referred to as ”near-hermetic” packaging [169]. A full hermetic packaging is
obtained using a metallic bonding scheme, however, at the cost of the relatively noncomplicated method to provide the spacing between the substrates using a polymer
ring.
The two substrates interact with each other both electrically and mechanically.
Yet, there are no vertical electrical interconnection vias which allow to access the
switch control electrodes on the switch substrate via the network substrate. The
lack of this vias is the main reason, why the first test structures were assembled
manually on chip-level: both the switch substrate (top part) and the network substrate
(bottom part) contain switch electrodes with contact pads. For the assembly the
switch substrate is flipped over and placed onto the spacer on the network substrate.
Now the electrode contact pads of the switch substrate cannot be accessed from the
top anymore and so far there are no vertical electrical interconnection vias allowing to
link the electrodes on the switch substrate to contact pads on the network substrate.
The chosen contacting method is illustrated in figure 37: the device is assembled on
chip-level with an overlap between the rims of the two substrates and the contact
pads are accessed from the top and from the bottom using probe needles. For the
early prototypes, this method was sufficient to evaluate the feasibility of the crossbar
switches, but future devices should contain vertical electrical interconnections.
54
Wafer-level heterogeneous integration of MEMS actuators
Figure 38. Picture showing a MEMS crossbar switch after the assembly. A weight was
necessary to force down the top part, counteracting the spring forces of the membranes, as
illustrated in the left-hand inset. Then, the top part with the switches was aligned to the
bottom part and finally the two substrates were fixated with epoxy droplets in the corners.
The moveable parts of the switches are released prior to integration: the membranes are fabricated by plasma enhanced chemical vapor deposition (PECVD) of
silicon nitride on a polymer sacrificial layer. Assisted by etch holes in the membrane,
the polymer was removed in a plasma ashing step. As a consequence, the thin membranes are not mechanically supported during the integration of the switch substrate,
potentially damaging the membranes. However, the manual assembly has shown that
the membranes of the switches are surprisingly mechanically robust. As an example:
during the manual assembly, the top part with 400 silicon nitride membranes with a
thickness of 1 μm was placed on the bottom part. Because of large stress gradients in
the membranes the deflection of the free ends were much larger than the thickness of
the spacer ring and the top part had to be forced down onto the spacer ring using a
weight as shown in figure 38. In this state, the switches were manually aligned (alignment accuracy ±5 μm), with considerable friction between the tips of the strained
membranes and the surface of the bottom part. Despite the friction and the resulting
strains and stresses, in most cases all the membranes survived the procedure.
The stress gradients in the membranes are high, resulting in high actuation voltages especially for the 20×20 array (∼ 95 V) with shorter membranes and smaller
electrode area. Therefore the PECVD process should be optimized to allow for more
control over the stress gradients in the deposited films. Furthermore, when depositing
the metal film for the membrane electrode additional stresses are induced between the
metal and the membrane. Also, the devices got very hot during the removal of the
sacrificial layer underneath the membranes in an oxygen plasma . The combination of
heat and plasma influences the stresses in the silicon nitride membranes [170,171,172],
which issues should be further investigated.
Most of the switches failed to provide a good ohmic contact. Assumed reasons are
5 AUTOMATED MAIN DISTRIBUTING FRAMES WITH S-SHAPED
ACTUATOR SWITCHES
55
that the contact force was not high enough and the surfaces of the switch contacts
were contaminated since the switch was not assembled in a cleanroom environment.
In future devices, the contact force can be increased by lowering the stress-gradient
in the membrane and by local stiffening of the membrane to improve the transfer of
the electrostatic force between the surrounding electrodes to the switch contacts. To
avoid contamination, future devices should be assembled in the cleanroom, ideally on
wafer-level and after a short plasma cleaning of the surfaces.
The actuators were very susceptible to stiction, which was most probably caused
by electrostatic stiction between the electrodes. In general, electrostatic stiction is
an issue for electrostatic actuators where dielectric layers are involved. Charges are
injected and trapped in the dielectric layers, thereby causing an electrostatic attraction. In the MEMS crossbar switch, charges can be trapped in the silicon nitride of
the membranes and the electrical isolation layers as well as in the large amount of
BCB underneath the bottom electrodes. Consequently, in future devices the amount
of dielectric materials should be kept at a minimum.
5.5
Outlook
Currently a second generation of MEMS crossbar switches is under development,
which features arrays of 20×20 double-switches on a silicon footprint of only 12×12 mm2 .
In the revised fabrication no complicated electroplating of thick metal layers is necessary and the overall stress gradients in the membranes can be controlled using a
stress-compensation design. The chip is fully hermetically packaged by metallic waferbonding and contains vertical electrical interconnection between the two substrates
as well as through-silicon vias through the network substrate to allow for hybrid integration as introduced in section 3. To obtain a more reliable operation of the switches
all polymers are removed to reduce the risk of electrostatic stiction caused by trapped
charges.
56
Wafer-level heterogeneous integration of MEMS actuators
6
6
SUMMARIES OF THE APPENDED PAPERS
57
Summaries of the Appended Papers
Paper 1: Out of plane knife gate microvalves for controlling large gas flow
This paper considers design issues for microvalves for large gas flow control. It
introduces out-of-plane knife-gate microvalves as a novel design concept and a
proportional microvalve concept for pressure control applications. The design of
three different actuator-gate configurations and first prototypes are presented.
The first valve prototypes feature thermal silicon–aluminum bimorph actuators and the pressure-flow performance per chip area of the demonstrator valve
presented is greatly increased using out-of-plane actuation and an out-of-plane
orifice. The characterization of the actuators and of the pressure-flow performance is presented.
Paper 2: Wafer-scale manufacturing of bulk shape memory alloy microactuators based
on adhesive bonding of Titanium-Nickel sheets to structured silicon wafers
This paper presents a concept for the wafer-scale manufacturing of microactuators based on the adhesive bonding of bulk shape memory alloy (SMA) sheets
to silicon microstructures. Wafer-scale integration of a cold-state deformation
mechanism is provided by the deposition of stressed films onto the SMA sheet. A
concept for heating of the SMA by Joule heating through a resistive heater layer
is presented. Critical fabrication issues were investigated, including the coldstate deformation, the bonding scheme and related stresses and the TitaniumNickel (TiNi) sheet patterning. Novel methods for the transfer stamping of adhesive and for the handling of the thin TiNi sheets were developed, based on the
use of standard dicing blue tape. First demonstrator TiNi cantilevers, waferlevel adhesively bonded on a microstructured silicon substrate, were successfully
fabricated and evaluated. Intrinsically stressed silicon dioxide and silicon nitride
were deposited to deform the cantilevers in the cold state and the resulting tip
deflections were evaluated.
Paper 3: Design and wafer-level fabrication of SMA wire microactuators on silicon
This paper reports on the fabrication of micro actuators through wafer-level
integration of pre-strained SMA wires to silicon structures. In contrast to
previous work, the wires are strained under pure tension, and the cold state
reset is provided by single crystalline silicon cantilevers. The fabrication is
based on standard MEMS manufacturing technologies and enables an actuation
scheme featuring high work densities. A mathematical model is discussed, which
58
Wafer-level heterogeneous integration of MEMS actuators
provides a useful approximation for practical designs and allows analyzing the
actuators performance. Prototypes have been tested and the influence of constructive variations on the actuator behavior is theoretically and experimentally
evaluated. The test results are in close agreement with the calculated values,
and show that the actuators feature displacements among the highest reported.
Paper 4: Localized removal of the Au-Si eutectic bonding layer for the selective release of microstructures
This paper presents and investigates a novel technique for the footprint and
thickness-independent selective release of Au–Si eutectically bonded microstructures through the localized removal of their eutectic bond interface. The technique is based on the electrochemical removal of the gold in the eutectic layer and
the selectivity is provided by patterning the eutectic layer and by proper electrical connection or isolation of the areas to be etched or removed, respectively.
The gold removal results in a porous silicon layer, acting similar to standard
etch holes in a subsequent sacrificial release etching. The paper presents the
principle and the design requirements of the technique. First test devices were
fabricated and the method successfully demonstrated. Furthermore, the paper
investigates the release mechanism and the effects of different gold layouts on
both the eutectic bonding and the release procedure.
Paper 5: Single-chip MEMS 5×5 and 20×20 double-pole single-throw switch arrays
for automating telecommunication networks
This paper reports on microelectromechanical (MEMS) switch arrays with 5×5
and 20×20 double-pole single-throw (DPST) switches embedded and packaged
on a single chip, which are intended for automating main distribution frames
in copper-wire telecommunication networks. Whenever a customer requests a
change in his telecommunication services, the copper-wire network has to be
reconfigured which is currently done manually by a costly physical re-routing
of the connections in the main distribution frames. To reduce the costs, new
methods for automating the network reconfiguration are sought after by the network providers. The presented devices comprise 5×5 or 20×20 double switches,
which allow us to interconnect any of the 5 or 20 input lines to any of the 5 or 20
output lines. The switches are based on an electrostatic S-shaped film actuator
with the switch contact on a flexible membrane, moving between a top and a
bottom electrode. The devices are fabricated in two parts which are designed to
be assembled using selective adhesive wafer bonding, resulting in a wafer-scale
package of the switch array. The on-chip routing network consists of thick metal
lines for low resistance and is embedded in bencocyclobutene (BCB) polymer
layers.
Paper 6: Row/Column addressing scheme for large electrostatic actuator MEMS
switch arrays and optimization of the operational reliability by statistical analysis
This paper investigates the design and optimization of a row/column addressing scheme to individually pull in or pull out single electrostatic actuators in
an N2 array, utilizing the electromechanical hysteresis behavior of electrostatic
6
SUMMARIES OF THE APPENDED PAPERS
59
actuators and efficiently reducing the number of necessary control lines from N2
complexity to 2N. This paper illustrates the principle of the row/column addressing scheme. Furthermore, it investigates the optimal addressing voltages
to individually pull in or pull out single actuators with maximum operational
reliability, determined by the statistical parameters of the pull-in and pull-out
characteristics of the actuators. The investigated addressing scheme is implemented for the individual addressing of cross-connect switches in a microelectromechanical systems 20×20 switch array, which is utilized for the automated
any-to-any interconnection of 20 input signal line pairs to 20 output signal line
pairs. The investigated addressing scheme and the presented calculations were
successfully tested on electrostatic actuators in a fabricated 20×20 array. The
actuation voltages and their statistical variations were characterized for different subarray cluster sizes. Finally, the addressing voltages were calculated and
verified by tests.
60
Wafer-level heterogeneous integration of MEMS actuators
7
7
CONCLUSIONS
61
Conclusions
This thesis presents methods for the wafer-level integration of TiNi and S-shaped electrostatic actuator switches to functionalize MEMS devices. The integration methods
are based on concepts of heterogeneous integration. Using the developed integration
methods, TiNi actuators are integrated with a new type of microvalves, drastically
improving their performance. The microvalves are knife gate microvalves and introduced in this thesis. The S-shaped actuator switches are arranged in arrays and
integrated with a signal line routing network, forming a MEMS crossbar switch.
In more detail, the developed TiNi integration methods:
– allow to wafer-level integrate bulk TiNi sheets and wires, together with mechanisms for the cold-state reset, avoiding the complicated monolithic integration
and the pic-and-place hybrid integration. First actuator test structures were
fabricated and evaluated, showing promising results. Heating with integrated
heaters is shown for the sheet based actuators;
– provide techniques for handling the TiNi prior to integration. The TiNi sheets
are patterned prior to integration to avoid exposure of the target structures to
the very aggressive TiNi etchant. The thin TiNi wires are oriented and strained
before they are transferred to the target substrate.
– are discussed, including initial tests of alternative approaches.
The developed knife gate microvalves:
– feature a gate moving perpendicular to the flow, reducing the demands on the
actuator. The gate moves out-of-plane to control an in-plane gas flow, minimizing the required silicon footprint area and the related costs;
– display excellent pneumatic performance in relation to the consumed silicon
footprint area, however, the first test structures required external actuation
since the integrated actuators failed;
62
Wafer-level heterogeneous integration of MEMS actuators
For combining knife gate microvalves and TiNi actuators, the thesis shows:
– first designs for integrating TiNi sheets and wires with silicon knife gate microvalves, including a method for the localized release of eutectically bonded,
thick silicon microstructures to allow their manipulation by the integrated actuators;
– first experiments displaying outstanding pneumatic performance of the valve in
relation to the consumed silicon footprint area.
– a discussion and outlook on future work, including a scheme for packaging of
the valve.
The MEMS crossbar switch:
– is intended as core switch unit for automating parts of copper-wire telecommunication networks;
– consists of two parts, with one part containing signal input and output lines
crossing each other and the other part containing an array of S-shaped actuator
switches. After integrating the switch array on the signal lines using adhesive
bonding, the switches allow to interconnect the input to the output lines in any
combination desired;
– features integrated encapsulation of the switch arrays;
– contains up to 20×20 = 400 crosspoint switches on an area of 12×16 mm2 .
First prototypes were fabricated and evaluated, proofing the feasibility of the
concept;
– requires a method for the individual addressing of selected switches. The thesis
presents the concept of row/column addressing, drastically reducing the necessary amount of control wires. The reliability of the addressing is maximized
by determining the addressing voltages based on the statistical deviation of the
actuation voltages across the array.
– is discussed and an outlook is given, based on current developments of a second
generation of MEMS crossbar switches.
7
CONCLUSIONS
(a)
63
(b)
(c)
Figure 39. Figure illustrating my gratitude to family, friends and colleagues in three
different languages: (a) Swedish, (b) English and (c) German.
Acknowledgments
This work was financially supported by Pondus Instruments AB, by Network Automation mxc AB, by Vinnova (the Swedish Agency for Innovation Systems) through the
Summit and the Forska&Väx framework, and by the European Commission through
the sixth framework programme funded project Q2M.
As all big projects, the project ’PhD’ requires the support of so many other people.
Figure 39 expresses what I want to say to you:
First of all, my principal supervisor Professor Göran Stemme for allowing me to
start the PhD-adventure in his group, yet always being there to limit the adventure
part and giving advice when needed. I truly appreciate that your door was always
open to discuss major and minor issues, not only during my ups but mainly during
my downs.
My supervisors Wouter van der Wijngaart and Joachim Oberhammer for showing
me how the academic world and research works. Special thanks to Wouter for not
only being my supervisor, but for the feeling of being welcome in the early days and
for being a great friend who helped during those downs.
Frank Niklaus for sorting out the worst in the integration section of this thesis
and for always having an open-minded approach to things.
Björn Samel for being a true friend, the great atmosphere in our ’German office’
and for giving me the assurance that I’m not the only German fighting against Swedish
windmills. Thank you for our friendship !
Another good friend, Niklas Sandström, for showing me the Swedish way of life
and of course for all the wonderful company when sharing hotel rooms (not like it
sounds) and when enjoying our ’matlåda’ together.
Henrik Gradin for being a great room-mate and for the very fruitful work we did
together. You’re a smart one and I hope some of the smartness diffused to my side
of the office.
Kjell Norén for his shared interest in everything with motors and wheels and
for demonstrating what a real engineer is. Thanks for all your brilliant ideas and
prototypes which helped so much.
Petra Palmquist for proofreading this thesis without despairing over it.
Special thanks also to my former professors, Peter Pokrowsky and Patrick Klär
for the support in the early days of my PhD adventure and for the remaining good
contact.
Of course a big ’thank you’ also to all the other colleagues I had the pleasure
64
Wafer-level heterogeneous integration of MEMS actuators
to work with in my projects. Sjoerd Haasl for the guidance and great spirit in my
early PhD days. Prof. Manfred Kohl, Dr. Thomas Grund and Johannes Barth from
FZK Karlsruhe for our fruitful work in the Q2M project and for the honor of inviting
me as speaker to your seminar. Special thanks to Thomas and the ’Deutsche Eiche’
Johannes for the nice time beside those meetings and conferences. Dr. Jan Peirs and
Donato Clausi from KUL Leuven for the fruitful work in the Q2M project. Donato
- I am glad I met you and I hope we will remain friends. Thorbjörn Ebefors, Göran
Edström, Göran Lindvall and Jochen Walter from Silex Microsystems for the close
collaboration when developing the MEMS crossbar switch 2.0. Cecilia Aronsson for
introducing me to the cleanroom routines and to the Swedish language. Sveinbjörg
Ingvarsdottír, Jutta Müntjes and Gaspard Pardon for enduring my guidance during
your master thesis and placements.
Thanks to Erika Appel and Hanne Eklund for paving the way through KTHbureaucracy and enabling things.
All the work was performed in a great atmosphere, for which I want to thank
my colleagues from KTH-MST: Göran, Frank, Joachim, Wouter, Hans, Erika, Kjell,
Niclas, Mikael, Mikael, Mikael (yes, it’s a common Swedish name), Niklas, Fredrik
and Carl Fredrik, Adit, Farizah, Andreas, Martin, Kristinn, Gaspard, Umer, Zargham,
Hitthesh, Henrik, Nutapong, Staffan.
Besides work, there is also a personal life. I would like to thank all my friends
who helped me during this time by either distracting me from work or cheering me
up in hard times. You know who you are - Thank you !
My family - always there for me. Thank you to my parents Regina and Kurt
for demonstrating the values of honesty and reliability and how to be almost perfect
parents. I have not forgotten that you should be among the research funders in the
first paragraph of the acknowledgments. Thank you to my brother Andreas for being
my big brother. Thank you to my niece Charlotte for the smile when I think of you.
Last, but not least - the biggest thank you to my Love Juliane for loving me the way
I am.
Das war meine Thesis - das Spiel geht weiter ...
REFERENCES
65
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Wafer-level heterogeneous integration of MEMS actuators
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